Make student auth per-group

Created by: iainbryson

Ok, so here’s what I think we’ve decided:

• Student auth is now going to be on a per-course+group level
• Admin auth will be on a per-course level, as it is now — being admin (or author) for one group is sufficient to be admin/author for all groups in the course
• When students leave a group, they lose access to in-progress or handed in quizzes. But those quizzes still count in the stats

If that’s right then I can put that in an issue.