Withdraw from a course in WebLearn

Now also getting the booking_id from our requests to the user_course listing.
Added booking_id to the domain
Amended the Presentation link for course booking
Service layer for withdrawing a user from a presentation

Handles issues like users trying to withdraw from courses they aren't registered on etc.
Updated the 'BookCourse' view to handle both Booking and Withdrawing

This hopefully leads to a more resource oriented design. Dare I say RESTful? :shipit:
Renamed 'BookCourse' -> 'PresentationBooking'
Merge branch 'master' of github.com:ox-it/moxie-courses