mattdrayer/api-course-observer-role: Added new 'observer' role, plus bat...

...ch PUT operation

@chrisdodge -- course observer support for your review.  Had to make changes to both student and instructor apps in order for everything to work correctly.  Also included is a new PUT operation for batch updating of a user's roles (see users/views.py, UsersRolesList) -- @dsego, FYI on that.

Hmmm. One gap here.

Say one has both 'instructor' and 'staff' and we remove the 'instructor' role. User still should be a forum moderator. Maybe do a query here to see if after the delete, there's still a role which should keep this forum moderator bit set