Add OSRA Branches

PT Story: https://www.pivotaltracker.com/story/show/77972984

- [x] Added Branch model and seed data
- [x] Added Branch specs and factory
- [x] Added relationship between Sponsor and Branch
As a compromise, perhaps we can agree on db-level constraints for 1. no null for booleans, so that they contain either `true` or `false` (as one would expect), and not `true`, `false` or `nil`; and 2. uniqueness. Neither is a case of over-engineering but rather ensuring expected behaviour in the first instance, and taking care of something AR can't always manage in the second.

So, we'd forgo `null: false` in the current situation.