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_jobs.py

@@ -1,12 +1,16 @@
 import json
 
 import pytest
-from flask import url_for
 from bs4 import BeautifulSoup
+from flask import url_for
+from freezegun import freeze_time
+from tests.conftest import (
+    SERVICE_ONE_ID,
+    mock_get_notifications,
+    normalize_spaces,
+)
 
 from app.main.views.jobs import get_time_left
-from tests.conftest import SERVICE_ONE_ID, normalize_spaces, mock_get_notifications
-from freezegun import freeze_time
 
 
 def test_get_jobs_should_return_list_of_all_real_jobs(