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.
Could you write up instructions for what they'll need to put in the local_settings.py file on each server, to enable this package? And perhaps, for now, on the nalanda branch, we should assert that the package was enabled, to make sure they don't miss one? We can then revert the assert once they've done this round of updating, before we merge to other branches.