Added prototype DoneXBlock and AudioXBlock

@nedbat @cpennington @jarv Mind if I merge? The blocks are experimental -- but I believe they are also fairly innocuous. I'll probably have a few styling updates (LMS breaks some of the styling) and perhaps some code cleanup before I try them on live students, but I'd like to start experimenting with them in course design. In particular, DoneXBlock would be very nice to integrate with Maker Physics. It'd be nice to get them on Edge so I can start doing that.