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.
Looks like this was copied from @cpauya's earlier code, but couldn't the whole method body be replaced by the following?
`self.is_exam_mode = self.test_id and get_exam_mode_on() == self.test_id`
(in which case it might not deserve its own method, and could just go in `__init__`)