[bug 1045623] Overhaul rate limiting and update limits

* reworks all the code so that we're using django-ratelimit as the core
  of our throttling instead of django-ratelimit for some things and
  django-rest-framework's throttling stuff for other things
* move throttling-related things from the feedback app to the base app
* change throttling from 100/h to 50/h for all posting (api and non-api)
* add throttling to the hb posting of 50/h
* add double-submit throttling to the api posting of 1/10m
* change double-submit throttling for non-api posting to 1/10m

r?
if `already_limited == True` then we fall through to `self.throttle_success()`?