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_letters.py

@@ -1,7 +1,8 @@
+from functools import partial
+
 import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
-from functools import partial
 
 letters_urls = [
     partial(url_for, 'main.add_service_template', template_type='letter'),