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/test_errorhandlers.py

@@ -1,7 +1,7 @@
-from flask import Response, url_for
-from flask_wtf.csrf import CSRFError
 import pytest
 from bs4 import BeautifulSoup
+from flask import Response, url_for
+from flask_wtf.csrf import CSRFError
 from notifications_python_client.errors import HTTPError