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/add_service.py

@@ -1,33 +1,18 @@
-from flask import (
-    render_template,
-    redirect,
-    session,
-    url_for,
-    current_app
-)
-
-from flask_login import (
-    current_user,
-    login_required
-)
+from flask import current_app, redirect, render_template, session, url_for
+from flask_login import current_user, login_required
 from notifications_python_client.errors import HTTPError
 from werkzeug.exceptions import abort
 
+from app import (
+    billing_api_client,
+    invite_api_client,
+    service_api_client,
+    user_api_client,
+)
 from app.main import main
 from app.main.forms import CreateServiceForm
 from app.notify_client.models import InvitedUser
-
-from app import (
-    invite_api_client,
-    user_api_client,
-    service_api_client,
-    billing_api_client
-)
-
-from app.utils import (
-    email_safe,
-    is_gov_user
-)
+from app.utils import email_safe, is_gov_user
 
 
 def _add_invited_user_to_service(invited_user):