Convert FeedbackResource to DRF (bug 910556)

https://bugzilla.mozilla.org/show_bug.cgi?id=910556

Remaining issues:
- Tests don't pass because they except errors to be inside an 'error_message' key ; DRF returns them directly instead, so instead of:
```
{'error_message': {'error1': 'foo'},{'error_2': 'bar'}}
```
We have
```
{'error1': 'foo'},{'error_2': 'bar'}
```
We need to determine if that's a problem and either fix the code to continue wrapping errors like tastypie or fix the tests to expect errors to be returned like DRF does. IMHO since it's already returned with a 400 error code we don't need the wrapping tastypie does.

- `ThrottleTests` is tastypie specific. Looking at the code I'm not convinced it was useful however so I removed it instead of making a DRF-version. tbh I'm unsure what to test here, it's almost like testing DRF directly, which is redundant. Thoughts ?