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_user_profile.py

@@ -1,10 +1,9 @@
-import pytest
-import uuid
 import json
+import uuid
 
+import pytest
 from flask import url_for
 from notifications_utils.url_safe_token import generate_token
-
 from tests.conftest import api_user_active as create_user