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

app/notify_client/models.py

@@ -1,7 +1,7 @@
 from itertools import chain
-from flask_login import UserMixin, AnonymousUserMixin
-from flask import request, session
 
+from flask import request, session
+from flask_login import AnonymousUserMixin, UserMixin
 
 roles = {
     'send_messages': ['send_texts', 'send_emails', 'send_letters'],