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

app/status/views/healthcheck.py

@@ -1,8 +1,9 @@
-from flask import jsonify, request, current_app
-from app import (version, status_api_client)
-from app.status import status
+from flask import current_app, jsonify, request
 from notifications_python_client.errors import HTTPError
 
+from app import status_api_client, version
+from app.status import status
+
 
 @status.route('/_status', methods=['GET'])
 def show_status():