[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?
Yeah. I need to stick with POST and not change the url to make Gregg's life easier. That way he can blindly just build the JavaScript object and keep sending updates without having to worry about whether it's the first or not. Simplifies a bunch of code on his end which is pretty important for this project.

This comment should read "inside the AnswerSerializer, too, ...". I'll fix that since it makes it clearer to future me and others what I was thinking.