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_errors_for_csv.py

@@ -4,7 +4,6 @@ import pytest
 
 from app.utils import get_errors_for_csv
 
-
 MockRecipients = namedtuple(
     'RecipientCSV',
     [