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/main/views/choose_service.py

@@ -1,7 +1,8 @@
-from flask import (render_template, redirect, url_for, session)
-from flask_login import login_required, current_user
-from app.main import main
+from flask import redirect, render_template, session, url_for
+from flask_login import current_user, login_required
+
 from app import service_api_client
+from app.main import main
 from app.notify_client.service_api_client import ServicesBrowsableItem
 from app.utils import is_gov_user