Course keys use key introspection

@brianhw, @watsonemily, @srpearce, and @stroilova, please review.
Question: is there an impact on CommentThread.commentable_id for
discussion threads?
this isn't an event, it's the name of a column in a SQL table. (the other file in this PR has events that include a ``course_id`` field.) "the course_id" here is a shortened version of "the value stored in the ``course_id`` column" and would be widely understood as such by researchers.