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/forgot_password.py

@@ -1,11 +1,9 @@
-from flask import (
-    render_template,
-)
+from flask import render_template
 from notifications_python_client.errors import HTTPError
 
+from app import user_api_client
 from app.main import main
 from app.main.forms import ForgotPasswordForm
-from app import user_api_client
 
 
 @main.route('/forgot-password', methods=['GET', 'POST'])