Change permissions to be per course #335

1. Allow permission to be applied to all courses
2. But also allow to limit the permission to one course or more

As an example, user A should be able to VIEW the solution of students in course J.
This is NOT necessarily implies that user A should be able to:

1. Comment on exercises in course J
2. VIEW the solutions of students in course K.
3. User B should be able to VIEW the solutions in either courses, J or K.