Displays release date of unit in sidebar

@andy-armstrong @cahrens please review.
Thanks @benmcmorran. As we discussed, I hadn't realized that sections always have explicit release dates set, presumably so you don't accidentally release your new section when you create it in a live course.

I think the one set of combinations that is missing then is asking a sequential for its release date source. You don't need this on the unit page, but I'll need it for the course outline.