Don't show some UI elements depending on the value of TURN_OFF_MOTIVATIONAL_FEATURES

Solves #2167. Does two things:
1. Add `TURN_OFF_MOTIVATIONAL_FEATURES` and the `Nalanda` package
2. Toggle off some UI features if the former is `True`.

Plus some other tweaks.

@jamalex please review.
I agree! We definitely should move this to an ab_testing app. Adding a TODO-BLOCKER.