First pass implementation of Store App

This implements the initial store app.

Also adds a signal for listening to student exam unsetting in order to trigger a 'gift card' transaction.

Summary of changes:
* Add store app.
* Add models for store transactions and store items.
* Add custom signal for exam unset.
* Add receiver for exam unset signal.
* Add practice attribute to Test object.
* Only trigger signal on practice tests.
* Add API endpoints for StoreItems and StoreTransactionLogs (the latter is restricted to authorized users only).
* Modifies total point reporting in statusModel API endpoint to include transaction logs.
* Prevents videos awarding points in Nalanda config.
* Parameterizes points reward for exercises based on (currently mock) unit.
@aronasorman -- I'm going to merge this into nalanda-rct3 now. Since you moved stuff out of the ab_testing app in a parallel PR, can you incorporate QUIZ_REPEATS into the new location in yours to make sure it doesn't break things when it's merged?