Errors unless `FORUM_MONGODB_DATABASE` and `FORUM_MONGODB_CLIENT_PARAMETERS` are defined

By default, we had intended for sites running master to require no changes at all to their configuration to keep the old experience. We currently break this, as @regisb notes in this post. The code is currently doing this because we sometimes need to derive the course_id to know which code path to take, and that's not always available from the request itself.

Is it feasible to add checking to forum.backends.mongodb.api.get_course_id_by_thread_id to explicitly check to see if FORUM_MONGODB_DATABASE and FORUM_MONGODB_CLIENT_PARAMETERS exist, and return None if they don't? That should make the public api call get_course_id_by_thread return None. (And the same for get_course_id_by_comment.) When passing a course_id of None to the CourseWaffleFlag check, it should return the default value for the site as a whole. Which I think will do what we want in a backwards compatible way?

Would that work? Are there other places where we need to try to infer the course_id?

Start in forum/backends/mongodb/api.py at get_course_id_by_thread_id, then trace forum/api/threads.py at get_course_id_by_thread and the corresponding comment path. Check how missing FORUM_MONGODB_DATABASE and FORUM_MONGODB_CLIENT_PARAMETERS are handled, identify any other course_id inference points, and verify that absent configuration returns None so CourseWaffleFlag uses the site-wide default.