Nalanda RCT3 Cleanup

Summary of changes:
* Display total correct when quiz completed.
* Switch StoreItem over to non-Django model.
* Award points for questions answered correctly on quizzes (based on server side basepoints).
* Increases seed across all items in a test so that no two items in a test have the same seed.
If we can ensure that points_per_video is always defined on the server side, then we can get rid of the redundancy checks here.

Yeah, I think turn_off_points_for_videos and turn_off_points_for_exercises could both be collapsed to a zero setting for videos and exercises.