Updating the design of the DRF views for enrollments.

Consolidate the try/catch blocks, and change the views to be a Class Based View for the RESTful layer. 

@wedaly @ormsbee 
Yeah, re-reading this after vacation, I'm more inclined to just have an update_enrollment function that allows you to modify mode or is_active. That would not be a big change, what do you think? Then function optionally takes either, all, or none, updates or creates the enrollment.