Course keys use key introspection

@brianhw, @watsonemily, @srpearce, and @stroilova, please review.
Question: is there an impact on CommentThread.commentable_id for
discussion threads?
@brianhw , my thought was that the phrasing "began to use" was both descriptive of the current state of affairs (some new courses do, some don't) and also would survive the transition to all new courses using this format. Does that come across at all?