Removed my name from the CONTRIBUTORS file

I thought I was signing up to work on the calendar team. Apparently I
misunderstood what we were doing. I have contacted the leader of that
team.
Updated authorization, added utility function

- Moved roles into the auth_group table.
- Added code to automatically populate the auth_group table with
default roles.
- Added code to add all users except user 1 to the auth_membership
table as student.
 - Updated methods in default.py to use the new auth_membership
relations.
- Added decorators to methods in default.py to enforce authorization.
- Updated view default/course.html to use the new auth_group.role
value, as my updates broke it.