Better display of branches on a repository

[brianmoose]

Currently when viewing a repository we only show branches that have had the status set. We should probably show all branches that have had jobs run against it, regardless of the status.

For example, the repo page for libmesh doesn't have any branches listed since we only do jobs for PRs on libmesh. So the status for a branch never gets set. But it currently isn't easily possible to see all the events on libmesh. If there was a "master" branch button (which is what PRs are against) then we could get a listing.