Backbone version of the course outline page

STUD-1726

Converted the course outline page to Backbone so as to support dynamic client-side updating.
@benmcmorran is touching this code in his PR as well. I do think we need to handle the case of edited_by not being an ID In the User table. So if User.objects.get(id=xblock.edited_by) doesn't find a user, perhaps just pass None? We definitely don't want "Invalid username", which will show in the UI.