Extend preview to support cohorted courseware

TNL-651.

Some code review has already been done on #6258. See that PR for sandbox information.
I agree. I've renamed both these methods using the phrase group_info rather than group_id, which hopefully implies that more than just a group id is returned. I decided to return the group id first in both cases, so I updated the original method to match. @cahrens, hopefully this addresses your concern with the old name.