Confusing UX around setting Github sources

[Marak]

We've received a few reports of difficulties setting the service source code to a Github repo ( public or private ).

We've landed #268 which now grants the ability for private repos, but the overall experience is still a bit confusing and error prone.

We need to improve the user interface to validate github resource properties the on the client, as well as give better error messages. stackvana/microcule@2e4c2fa and fc479f2 should also help ( but have not been deployed yet )