Enrollment API Cleanup

This PR will capture the following work
* Update to version 1
* Change URL scheme to match other APIs
* Add Course Enrollment Detail entry point
* Use Class Based DRF views

@ormsbee @wedaly This should be ready for review. 

Where are we preventing users from enrolling in paid courses?  I see validation logic in `api.py` that checks whether the course mode exists, but I'm not seeing where we check whether the course mode is paid.

We *do* need to support enrollment in paid modes, but only for server-to-server requests.