Move primary branch identification into the API from the UI. This simplifies a lot of UI code, but for now requires primary (default) branch identification to be static in the API. At a future point in time, this could be stored in the database...

Summary: ...(and no longer needs to be static).

Reviewers: cramer

Reviewed By: cramer

Differential Revision: https://tails.corp.dropbox.com/D63007