Ports changes to models and model tests along with some settings changes...

... from feature/tracks_activation
At the moment, the requirements for finalization is that it's a course-wide setting.  Once you finalize tracks for the course, you can't add or remove tracks, and before you finalize tracks for a course, students can't be added to them.  This is to protect against the statistical validity issues arising from changing the number of tracks after students have been added.