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/test_utils.py

@@ -1,22 +1,22 @@
-from pathlib import Path
-from io import StringIO
 from collections import OrderedDict
 from csv import DictReader
+from io import StringIO
+from pathlib import Path
 
-from freezegun import freeze_time
 import pytest
+from freezegun import freeze_time
+from tests.conftest import fake_uuid
 
 from app.utils import (
+    GovernmentDomain,
+    Spreadsheet,
     email_safe,
+    generate_next_dict,
     generate_notifications_csv,
     generate_previous_dict,
-    generate_next_dict,
-    Spreadsheet,
-    get_letter_timings,
     get_cdn_domain,
-    GovernmentDomain,
+    get_letter_timings,
 )
-from tests.conftest import fake_uuid
 
 
 def _get_notifications_csv(