[bug 1092299] Implement Heartbeat v2 API

* implements the v2 API relying more on the DRF serializer validation
* implements new tests
* fixes issues with the model by nixing the db table and recreating it
* adds a survey admin

r?
Gregg will understand this, but you're right--outside of him and I, no one else will. It's referring to how v1 worked. You could provide additional post data and those would be swept up and stuck in a JSON field that was called "extra":

https://github.com/mozilla/fjord/blob/9d311ca86f90cb031c50337d2cd428bda7655d81/fjord/heartbeat/views.py#L54

https://github.com/mozilla/fjord/blob/master/docs/api.rst#required-fields-1

The original v2 spec called this field "slop" instead of "extra". I changed that aspect last night and renamed it to "extra".