Course keys use key introspection

@brianhw, @watsonemily, @srpearce, and @stroilova, please review.
Question: is there an impact on CommentThread.commentable_id for
discussion threads?
I believe that the format is "block-v1:edX+DemoX+Demo_2014+type@problem+block@303034da25524878a2e66fb57c91cf85".  There is no longer a reference here to i4x://.