Add feed collection models, serializer, and viewset.

r?

- [ ]  @ngokevin r-
- [x] @ngokevin approved
- [x] @ngokevin verified
- [x] @ngokevin certified
- [x] @ngokevin signed off like the Declaration of Independence
- [x] @ngokevin :+1: 
- [x]  this patch has been approved by @ngokevin
- [x]  robust approval

TODO:
- [ ] Allow apps to be groupable
- [x] Allow apps to be created/edited as a property of the resource itself, rather than at a subresource.