Base feed collection classes; Editorial Brand model, serializer, and API endpoints.

WIP, still a few things I need to do:

- [x] Write tests for the new permissions class
- [x] Fix a broken ordering test (which we may not need at all?)
- [x] Rework FeedBrand fields to work with strings instead of PKs: layout, type
- [ ] Write docs for new APIs
- [ ] Finish docstrings

Getting this out there ASAP to unblock Kevin.

r? @ngokevin