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/notify_client/test_notify_admin_api_client.py

@@ -1,13 +1,12 @@
 import uuid
-import pytest
 from unittest.mock import patch
 
+import pytest
 import werkzeug
-
 from tests import service_json
 from tests.conftest import api_user_active, platform_admin_user
-from app.notify_client import NotifyAdminAPIClient
 
+from app.notify_client import NotifyAdminAPIClient
 
 SAMPLE_API_KEY = '{}-{}'.format('a' * 36, 's' * 36)