Convert FeedbackResource to DRF (bug 910556)

https://bugzilla.mozilla.org/show_bug.cgi?id=910556

Notable changes due to the conversion:
- Errors used 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 determined that because it's always associated to a 400 HTTP Status code, putting the errors in an 'error_message' key is superfluous. This is a backwards-incompatible change but shouldn't have much impact for now.

- `ThrottleTests` is tastypie specific. We removed it from the tests because it wasn't very useful anyway. We might have to revisit this issue in the next API we switch to DRF.