Course participants

Implementing #185 
Unless I'm mistaken (quite possible), if you assign each status a number such that `completed` is the highest, you can do something like:

    SELECT user_id, exercise_name, MAX(status) FROM submissions WHERE course_id = ? GROUP BY user_id, exercise_name;

With an index on user_id, course_id, exercise_name and status, this might be reasonably fast.

I'm open to better suggestions. That's just the first that came to mind.