User Tours are walkthroughs we are able to give in our frontends.
This sets up the backend support for them by creating the model,
setting up the initial backfill, adds in a signal handler to init
the UserTour model on User creation, and sets up some endpoints
to get user tour information and update it. It is also being
initialized with a waffle flag to control the rollout. The flag is
intended to control all tours and not allow for opting into only some tours.
