Simplify data model and add method for retrieving notes for a release.

- Removes the Product, Channel, and Tag models and replaces them with
  IntegerFields with choices.
- Store version number on Release only, as a string.
- Instead of using weird math to figure out what range of releases a
  note should appear on, use an m2m relationship to explicitly list it.
- Add method to Release for retrieving notes for that release.
- Update clients to support m2m relations.

Contains an excessive amount of migrations because we're going to remove
them soon anyway, so it's no big deal.