New Feature: optionally use Fitbit's subscription API to subscribe to user data

Subscribing to user data allows us to get near realtime updates from Fitbit when there is new data and store that data locally.
Another related change is the addition of a generic AJAX endpoint for getting fitbit data other than steps, and the deprecation of the current steps specific endpoint.
I documented how to use the new feature and wrote a couple tests for the subscription endpoint and celery worker.