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_notifications.py

@@ -1,10 +1,14 @@
-from freezegun import freeze_time
-from flask import url_for
 from functools import partial
-import pytest
 
+import pytest
+from flask import url_for
+from freezegun import freeze_time
 from notifications_utils.template import LetterImageTemplate
-from tests.conftest import mock_get_notification, SERVICE_ONE_ID, normalize_spaces
+from tests.conftest import (
+    SERVICE_ONE_ID,
+    mock_get_notification,
+    normalize_spaces,
+)
 
 
 @pytest.mark.parametrize('notification_status, expected_status', [