Fix for #187 on central: show thumbnails for coach reports
Initializes store app.
Adds models for store app.
Updates model fields for StoreItem model.
Adds store to installed_apps.
Adds initial migrations for store models.
Modifies set_exam_mode_on to be useable from API call as well as tests.
Fires signal for exam_mode being unset.
Sets up receiver to listen to exam_unset signal.
Implements first pass of transaction creation on exam unset.
TODO-BLOCKER: Tie into a/b testing framework.
Adds 'practice' flag to Test object.
Restricts signal issuing to practice tests.
Moves all set_exam_mode logic inside utils function.
Adds tastypie API endpoints for StoreItem and StoreTransactionLog models.
Updates points display to include StoreTransactions.
Implements first pass of nerfing video points and parameterizing exercise points based on unit.
TODO: Update based on a/b testing framework implementation and unit setting.
Modifies exercise basepoints depending on selected unit.
Merge pull request #2295 from rtibbles/output_reward

First pass implementation of Store App
Merge pull request #2258 from dylanjbarth/central-127

Fix for #187 on central: show thumbnails for coach reports
Initial implementation for #2311 without using django's model.
Merge branch 'feature/2311-Advance-to-the-next-unit-functionality' of https://github.com/cpauya/ka-lite into feature/2311-Advance-to-the-next-unit-functionality