Deprecate `.model` attribute on views

Closes #1782

- [x] Update quickstart, and README examples.
- [x] Raise deprecation if accessed instead of `.queryset` or `.get_serializer_class`.
- [x] Deprecate `DEFAULT_MODEL_SERIALIZER_CLASS` setting.
- [x] Add to release notes.
- [x] Note deprecation in relevant parts of the documentation.