Enforce order and style of imports

Done using isort[1], with the following command:
```
isort -rc ./app ./tests
```

Adds linting to the `run_tests.sh` script to stop badly-sorted imports
getting re-introduced.

Chosen style is ‘Vertical Hanging Indent’ with trailing commas, because
I think it gives the cleanest diffs, eg:
```
from third_party import (
    lib1,
    lib2,
    lib3,
    lib4,
)
```

1. https://pypi.python.org/pypi/isort

tests/app/main/views/test_manage_users.py

@@ -1,19 +1,21 @@
 import copy
+
 import pytest
-from flask import url_for
 from bs4 import BeautifulSoup
+from flask import url_for
+from tests.conftest import (
+    SERVICE_ONE_ID,
+    active_user_manage_template_permission,
+    active_user_no_mobile,
+    active_user_view_permissions,
+    active_user_with_permissions,
+    normalize_spaces,
+)
+from tests.conftest import service_one as create_sample_service
+
 import app
 from app.notify_client.models import InvitedUser
 from app.utils import is_gov_user
-from tests.conftest import service_one as create_sample_service
-from tests.conftest import (
-    normalize_spaces,
-    SERVICE_ONE_ID,
-    active_user_with_permissions,
-    active_user_view_permissions,
-    active_user_no_mobile,
-    active_user_manage_template_permission,
-)
 
 
 @pytest.mark.parametrize('user, expected_self_text, expected_coworker_text', [