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 `points_per_video` is set to 0, the `||` might force it back up to 750, right?

Also, looking at this, we might be able to reduce the redundancy here, getting rid of `turn_off_points_for_videos` and just setting `points_per_video` to 0 (and I guess, then, removing the client-side default of 750, just relying on there being a dynamic settings value.