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.
Since granular translations won't work well for many languages, we should probably have a way to do `sprintf`-style i18n-friendly interpolation from within the `_` Handlebars helper. I can take a look at that if nobody else jumps on it.