Course participants

Implementing #185 
On my desktop, with the mooc production DB dump, this takes around 30 seconds for the k2014-mooc course. I'd really like to avoid adding more really slow queries to TMC.

Would the following make sense?
- Let this only return a list of participants and their newest submission IDs.
- Load a user's exercise statuses on demand, and cache them either on your side or maybe in TMC until the submission ID changes.
- Consider having the per-user URL support [ETags](http://en.wikipedia.org/wiki/HTTP_ETag).