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_service_api_client.py

@@ -1,8 +1,8 @@
 import pytest
+from tests.conftest import SERVICE_ONE_ID, fake_uuid
 
 from app import service_api_client
 from app.notify_client.service_api_client import ServiceAPIClient
-from tests.conftest import fake_uuid, SERVICE_ONE_ID
 
 
 def test_client_posts_archived_true_when_deleting_template(mocker):