Add flake8 linting to project

The GDS Way™[1] recommends using Flake8 to lint Python projects.

This commit takes the Flake8 config from Digital Marketplace API[2] and
removes the bits we don’t need.

It changes the `max_complexity` setting to 14, which is the most complex
code we have in this repo currently (we shouldn’t be writing code _more_
complex than what we already have).

This commit also fixes the errors found by Flake8, which includes 6(!)
tests which were never getting run because they had the same names as
existing tests.

Here is a full list of the errors that were found and fixed:
```
./app/__init__.py:2:1: F401 're' imported but unused
./app/__init__.py:4:1: F401 'json' imported but unused
./app/__init__.py:8:1: F401 'dateutil' imported but unused
./app/__init__.py:11:1: F401 'flask.escape' imported but unused
./app/__init__.py:41:1: F401 'app.proxy_fix' imported but unused
./app/__init__.py:129:5: F821 undefined name 'proxy_fix'
./app/__init__.py:221:19: F821 undefined name 'highlight'
./app/__init__.py:221:35: F821 undefined name 'JavascriptLexer'
./app/__init__.py:221:54: F821 undefined name 'HtmlFormatter'
./app/config.py:2:1: F401 'datetime.timedelta' imported but unused
./app/event_handlers.py:2:1: F401 'flask_login.current_user' imported but unused
./app/utils.py:11:1: F401 'dateutil.parser' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.two_factor' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.notifications' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.add_service' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.forgot_password' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.inbound_number' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.styleguide' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.organisations' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.letter_jobs' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.verify' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.conversation' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.api_keys' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.send' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.dashboard' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.jobs' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.manage_users' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.sign_in' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.sign_out' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.code_not_received' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.invites' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.platform_admin' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.providers' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.service_settings' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.index' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.new_password' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.user_profile' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.feedback' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.choose_service' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.templates' imported but unused
./app/main/__init__.py:5:1: F401 'app.main.views.register' imported but unused
./app/main/forms.py:12:1: F401 'wtforms.SelectField' imported but unused
./app/main/views/api_keys.py:37:29: E241 multiple spaces after ':'
./app/main/views/feedback.py:3:1: F401 'flask.flash' imported but unused
./app/main/views/feedback.py:122:17: E123 closing bracket does not match indentation of opening bracket's line
./app/main/views/inbound_number.py:1:1: F401 'flask.url_for' imported but unused
./app/main/views/inbound_number.py:1:1: F401 'flask.session' imported but unused
./app/main/views/inbound_number.py:1:1: F401 'flask.redirect' imported but unused
./app/main/views/inbound_number.py:1:1: F401 'flask.request' imported but unused
./app/main/views/inbound_number.py:13:1: F401 'flask.jsonify' imported but unused
./app/main/views/jobs.py:31:1: F401 'app.utils.get_template' imported but unused
./app/main/views/letter_jobs.py:1:1: F401 'datetime' imported but unused
./app/main/views/letter_jobs.py:6:1: F401 'app.format_datetime_24h' imported but unused
./app/main/views/manage_users.py:111:9: E123 closing bracket does not match indentation of opening bracket's line
./app/main/views/notifications.py:121:5: F841 local variable 'status_args' is assigned to but never used
./app/main/views/organisations.py:1:1: F401 'flask.request' imported but unused
./app/main/views/service_settings.py:77:9: E123 closing bracket does not match indentation of opening bracket's line
./app/main/views/service_settings.py:82:9: E123 closing bracket does not match indentation of opening bracket's line
./app/main/views/service_settings.py:420:13: E123 closing bracket does not match indentation of opening bracket's line
./app/main/views/sign_in.py:12:1: F401 'flask_login.confirm_login' imported but unused
./app/main/views/sign_in.py:17:1: F401 'app.service_api_client' imported but unused
./app/main/views/sign_in.py:62:13: E123 closing bracket does not match indentation of opening bracket's line
./app/main/views/templates.py:4:1: F401 'flask.json' imported but unused
./app/main/views/templates.py:17:1: F401 'notifications_utils.formatters.escape_html' imported but unused
./app/main/views/templates.py:23:1: F401 'app.utils.get_help_argument' imported but unused
./app/main/views/templates.py:64:13: E123 closing bracket does not match indentation of opening bracket's line
./app/notify_client/service_api_client.py:6:1: F401 '.notification_api_client' imported but unused
./app/notify_client/user_api_client.py:1:1: F401 'uuid' imported but unused
./app/notify_client/user_api_client.py:3:1: F401 'flask.session' imported but unused
./tests/__init__.py:1:1: F401 'csv' imported but unused
./tests/app/main/test_asset_fingerprinter.py:2:1: F401 'os' imported but unused
./tests/app/main/test_asset_fingerprinter.py:4:1: F401 'unittest.mock' imported but unused
./tests/app/main/test_asset_fingerprinter.py:98:9: F841 local variable 'string_with_unicode_character' is assigned to but never used
./tests/app/main/test_errorhandlers.py:2:1: F401 'flask.url_for' imported but unused
./tests/app/main/test_permissions.py:26:13: F841 local variable 'response' is assigned to but never used
./tests/app/main/test_placeholder_form.py:3:1: F401 'wtforms.Label' imported but unused
./tests/app/main/test_placeholder_form.py:11:10: F841 local variable 'req' is assigned to but never used
./tests/app/main/test_two_factor_form.py:10:67: F841 local variable 'req' is assigned to but never used
./tests/app/main/test_two_factor_form.py:23:65: F841 local variable 'req' is assigned to but never used
./tests/app/main/test_two_factor_form.py:37:48: F841 local variable 'req' is assigned to but never used
./tests/app/main/test_two_factor_form.py:51:67: F841 local variable 'req' is assigned to but never used
./tests/app/main/test_two_factor_form.py:65:67: F841 local variable 'req' is assigned to but never used
./tests/app/main/views/test_accept_invite.py:356:5: F841 local variable 'element' is assigned to but never used
./tests/app/main/views/test_activity.py:11:1: F811 redefinition of unused 'mock_get_notifications' from line 11
./tests/app/main/views/test_activity.py:18:1: F401 'datetime.datetime' imported but unused
./tests/app/main/views/test_activity.py:102:5: F841 local variable 'content' is assigned to but never used
./tests/app/main/views/test_activity.py:104:5: F841 local variable 'notification' is assigned to but never used
./tests/app/main/views/test_activity.py:337:5: F841 local variable '_notifications_mock' is assigned to but never used
./tests/app/main/views/test_activity.py:373:13: E126 continuation line over-indented for hanging indent
./tests/app/main/views/test_activity.py:378:9: E121 continuation line under-indented for hanging indent
./tests/app/main/views/test_activity.py:404:13: E126 continuation line over-indented for hanging indent
./tests/app/main/views/test_activity.py:407:9: E121 continuation line under-indented for hanging indent
./tests/app/main/views/test_api_keys.py:354:5: F841 local variable 'response' is assigned to but never used
./tests/app/main/views/test_conversation.py:5:1: F401 'bs4.BeautifulSoup' imported but unused
./tests/app/main/views/test_conversation.py:198:5: F841 local variable 'mock_get_inbound_sms' is assigned to but never used
./tests/app/main/views/test_dashboard.py:53:5: F841 local variable 'mock_template_stats' is assigned to but never used
./tests/app/main/views/test_dashboard.py:72:5: F841 local variable 'mock_template_stats' is assigned to but never used
./tests/app/main/views/test_jobs.py:2:1: F401 'uuid' imported but unused
./tests/app/main/views/test_jobs.py:3:1: F401 'urllib.parse.urlparse' imported but unused
./tests/app/main/views/test_jobs.py:3:1: F401 'urllib.parse.quote' imported but unused
./tests/app/main/views/test_jobs.py:3:1: F401 'urllib.parse.parse_qs' imported but unused
./tests/app/main/views/test_jobs.py:9:1: F401 'app.main.views.jobs.get_status_filters' imported but unused
./tests/app/main/views/test_jobs.py:10:1: F401 'tests.notification_json' imported but unused
./tests/app/main/views/test_letters.py:6:1: F401 'tests.service_json' imported but unused
./tests/app/main/views/test_notifications.py:5:1: F401 'app.utils.REQUESTED_STATUSES' imported but unused
./tests/app/main/views/test_notifications.py:5:1: F401 'app.utils.DELIVERED_STATUSES' imported but unused
./tests/app/main/views/test_notifications.py:5:1: F401 'app.utils.SENDING_STATUSES' imported but unused
./tests/app/main/views/test_notifications.py:5:1: F401 'app.utils.FAILURE_STATUSES' imported but unused
./tests/app/main/views/test_platform_admin.py:242:13: E126 continuation line over-indented for hanging indent
./tests/app/main/views/test_platform_admin.py:247:13: E126 continuation line over-indented for hanging indent
./tests/app/main/views/test_send.py:3:1: F401 'unittest.mock.Mock' imported but unused
./tests/app/main/views/test_send.py:18:1: F811 redefinition of unused 'mock_get_service' from line 18
./tests/app/main/views/test_send.py:18:1: F401 'tests.conftest.multiple_letter_contact_blocks' imported but unused
./tests/app/main/views/test_send.py:18:1: F401 'tests.conftest.no_sms_senders' imported but unused
./tests/app/main/views/test_send.py:18:1: F401 'tests.conftest.multiple_sms_senders' imported but unused
./tests/app/main/views/test_send.py:18:1: F401 'tests.conftest.no_letter_contact_blocks' imported but unused
./tests/app/main/views/test_send.py:102:5: F841 local variable 'response' is assigned to but never used
./tests/app/main/views/test_send.py:870:5: F841 local variable 'response' is assigned to but never used
./tests/app/main/views/test_send.py:1367:5: F841 local variable 'service_id' is assigned to but never used
./tests/app/main/views/test_send.py:1451:13: E126 continuation line over-indented for hanging indent
./tests/app/main/views/test_send.py:1620:80: E226 missing whitespace around arithmetic operator
./tests/app/main/views/test_send.py:1909:13: E126 continuation line over-indented for hanging indent
./tests/app/main/views/test_send.py:1912:9: E121 continuation line under-indented for hanging indent
./tests/app/main/views/test_service_settings.py:13:1: F811 redefinition of unused 'no_reply_to_email_addresses' from line 13
./tests/app/main/views/test_service_settings.py:13:1: F401 'tests.conftest.single_reply_to_email_address' imported but unused
./tests/app/main/views/test_service_settings.py:28:5: E123 closing bracket does not match indentation of opening bracket's line
./tests/app/main/views/test_service_settings.py:104:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:166:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:186:5: F841 local variable 'mocked_get_fn' is assigned to but never used
./tests/app/main/views/test_service_settings.py:217:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:237:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:257:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:307:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:340:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:466:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:555:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:615:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:719:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:874:5: F841 local variable 'page' is assigned to but never used
./tests/app/main/views/test_service_settings.py:902:5: F841 local variable 'page' is assigned to but never used
./tests/app/main/views/test_service_settings.py:954:5: F841 local variable 'page' is assigned to but never used
./tests/app/main/views/test_service_settings.py:986:5: F841 local variable 'page' is assigned to but never used
./tests/app/main/views/test_service_settings.py:1101:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1121:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1271:1: F811 redefinition of unused 'test_set_letter_contact_block_saves' from line 1189
./tests/app/main/views/test_service_settings.py:1433:5: F841 local variable 'page' is assigned to but never used
./tests/app/main/views/test_service_settings.py:1495:5: F841 local variable 'mocked_get_fn' is assigned to but never used
./tests/app/main/views/test_service_settings.py:1540:5: F841 local variable 'mocked_get_fn' is assigned to but never used
./tests/app/main/views/test_service_settings.py:1570:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1589:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1621:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1641:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1658:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1676:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1697:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1759:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_service_settings.py:1775:1: F811 redefinition of unused 'single_reply_to_email_address' from line 13
./tests/app/main/views/test_templates.py:3:1: F401 'uuid' imported but unused
./tests/app/main/views/test_templates.py:11:1: F401 'tests.conftest.mock_get_user' imported but unused
./tests/app/main/views/test_templates.py:514:1: F811 redefinition of unused 'mock_get_user' from line 11
./tests/app/main/views/test_templates.py:672:1: F811 redefinition of unused 'mock_get_user' from line 11
./tests/app/main/views/test_templates.py:795:1: F811 redefinition of unused 'mock_get_user' from line 11
./tests/app/main/views/test_templates.py:835:1: F811 redefinition of unused 'mock_get_user' from line 11
./tests/app/main/views/test_two_factor.py:67:13: E126 continuation line over-indented for hanging indent
./tests/app/notify_client/test_notification_client.py:79:5: F841 local variable 'mock_post' is assigned to but never used
```

1. https://gds-way.cloudapps.digital/manuals/programming-languages/python/linting.html#how-to-use-flake8
2. d5ab8afef4/.flake8

.flake8

@@ -0,0 +1,7 @@
+[flake8]
+# Rule definitions: http://flake8.pycqa.org/en/latest/user/error-codes.html
+# W503: line break before binary operator
+exclude = venv*,__pycache__,node_modules,cache
+ignore = W503
+max-complexity = 14
+max-line-length = 120