Backbone version of the course outline page

STUD-1726

Converted the course outline page to Backbone so as to support dynamic client-side updating.
This is the base class, so it could be any xblock that is added as a child. In the cases we have right now, the only case that this will get called is from the Unit Outline which has just a single "New Unit" button. The other situation is handled by the overriding method on CourseOutlineView which supports adding sections, subsections and units.