Stop using _external=True in tests

It looks like, by default, Flask no longer makes full URLs, for example
`https://example.com/path`. Instead it does `/path`. This will still
work fine, and if anything is better because it reduces the number of
bytes of HTML we are sending.

It won’t mean that requests go over `http` instead of `https` without
the protocol because we set the appropriate HSTS header here:
0c57da7781/ansible/roles/paas-proxy/templates/admin.conf.j2 (L11)

This commit changes all our tests to reflect that URLs no longer have
the protocol and domain in them. `_external=True` is Flask’s way of
saying whether a URL should be generated with the domain and protocol
(`True`) or without it (`False`).

Again, I can’t find the changelog or diff where this was introuduced,
but if you’d like to go spelunking then here’s a starting point:
50374e3cfe/src/flask/helpers.py (L192)

tests/app/main/views/test_manage_users.py

@@ -633,7 +633,6 @@ def test_edit_user_permissions(
         _expected_redirect=url_for(
             'main.manage_users',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         ),
     )
     mock_set_user_permissions.assert_called_with(
@@ -713,7 +712,6 @@ def test_edit_user_permissions_for_broadcast_service(
         _expected_redirect=url_for(
             'main.manage_users',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         ),
     )
     mock_set_user_permissions.assert_called_with(
@@ -762,7 +760,6 @@ def test_edit_user_folder_permissions(
         _expected_redirect=url_for(
             'main.manage_users',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         ),
     )
     mock_set_user_permissions.assert_called_with(
@@ -811,7 +808,6 @@ def test_cant_edit_user_folder_permissions_for_platform_admin_users(
         _expected_redirect=url_for(
             'main.manage_users',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         ),
     )
     mock_set_user_permissions.assert_called_with(
@@ -874,7 +870,6 @@ def test_edit_user_permissions_including_authentication_with_email_auth_service(
         _expected_redirect=url_for(
             'main.manage_users',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         ),
     )
 
@@ -1697,7 +1692,7 @@ def test_remove_user_from_service(
         'main.remove_user_from_service',
         service_id=service_one['id'],
         user_id=active_user_with_permissions['id'],
-        _expected_redirect=url_for('main.manage_users', service_id=service_one['id'], _external=True)
+        _expected_redirect=url_for('main.manage_users', service_id=service_one['id'])
     )
     mock_remove_user_from_service.assert_called_once_with(
         service_one['id'],
@@ -1778,7 +1773,6 @@ def test_edit_user_email_redirects_to_confirmation(
             'main.confirm_edit_user_email',
             service_id=SERVICE_ONE_ID,
             user_id=active_user_with_permissions['id'],
-            _external=True,
         ),
     )
     with client_request.session_transaction() as session:
@@ -1805,7 +1799,6 @@ def test_edit_user_email_without_changing_goes_back_to_team_members(
         _expected_redirect=url_for(
             'main.manage_users',
             service_id=SERVICE_ONE_ID,
-            _external=True
         ),
     )
     assert mock_update_user_attribute.called is False
@@ -1835,7 +1828,6 @@ def test_edit_user_email_can_change_any_email_address_to_a_gov_email_address(
             'main.confirm_edit_user_email',
             service_id=SERVICE_ONE_ID,
             user_id=active_user_with_permissions['id'],
-            _external=True
         ),
     )
 
@@ -1862,7 +1854,6 @@ def test_edit_user_email_can_change_a_non_gov_email_address_to_another_non_gov_e
             'main.confirm_edit_user_email',
             service_id=SERVICE_ONE_ID,
             user_id=active_user_with_permissions['id'],
-            _external=True
         ),
     )
 
@@ -1975,7 +1966,6 @@ def test_confirm_edit_user_email_changes_user_email(
         _expected_redirect=url_for(
             'main.manage_users',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         ),
     )
 
@@ -2087,7 +2077,6 @@ def test_edit_user_mobile_number_redirects_to_confirmation(
             'main.confirm_edit_user_mobile_number',
             service_id=SERVICE_ONE_ID,
             user_id=active_user_with_permissions['id'],
-            _external=True,
         ),
     )
 
@@ -2109,7 +2098,6 @@ def test_edit_user_mobile_number_redirects_to_manage_users_if_number_not_changed
         _expected_redirect=url_for(
             'main.manage_users',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         ),
     )
 
@@ -2189,7 +2177,6 @@ def test_confirm_edit_user_mobile_number_changes_user_mobile_number(
         _expected_redirect=url_for(
             'main.manage_users',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         ),
     )
     mock_update_user_attribute.assert_called_once_with(