Action required policy: extend tests with the new behavior
Action required policy: extend tests with the new behavior

Changed:
* adhering to `?next` query parameter
* throwing 404 when accessing the `action_required_policy` view by
  user who already agreed on that policy

The tests were adjusted for these changes.

swcarpentry/amy,swcarpentry/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,swcarpentry/amy,pbanaszkiewicz/amy