Also fixes failing Python unit tests

Expose a new endpoint to query a list of users based on a
role name (Moderator, Student, etc.) and a course_id.

This will initially be used by the notifier to send daily digest
messages to forum moderators.

The endpoint includes the key of the desired preference in the URL and
returns the list of users for whom the preference is set (regardless of
the value).

Also, refactor tests such that we're not deleting users or prefs

The new API uses Django REST Framework. For now, it is designed specifically
to support the use cases required by the forum digest notifier (not yet built),
with a goal of making it more generally useful over time.