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/two_factor.py

@@ -1,20 +1,22 @@
 import json
+
 from flask import (
-    render_template,
+    current_app,
+    flash,
     redirect,
+    render_template,
+    request,
     session,
     url_for,
-    request,
-    current_app,
-    flash
 )
-from flask_login import login_user, current_user
+from flask_login import current_user, login_user
+from itsdangerous import SignatureExpired
+from notifications_utils.url_safe_token import check_token
+
+from app import service_api_client, user_api_client
 from app.main import main
 from app.main.forms import TwoFactorForm
-from app import service_api_client, user_api_client
 from app.utils import redirect_to_sign_in
-from notifications_utils.url_safe_token import check_token
-from itsdangerous import SignatureExpired
 
 
 @main.route('/two-factor-email-sent', methods=['GET'])