Improve dev workflow for adding public repositories

Created by: nicksnyder

With the transition to external services, I removed the ability for site admins to add public repositories simply by visiting them and clicking a button. This is a case of some things getting better (external services give us a better data model for dealing with repository renames) and some things getting worse.

I tried to mitigate this workflow from getting worse, but decided that it added too much complexity.

My hypothesis is that this particular workflow that got worse is most commonly a development workflow and not an actual customer workflow. I think in the short term it is actually good for our team to get familiar with how customers configure external services (rather than providing a short cut). In the longer term, we might be able to mitigate this development pain by doing other things (like pre-loading a set of public repos in a dev environment).