Add capacity to retrofit GitHub Application to existing GitHub repos and Pantheon sites

[stevector]

Currently the GitHub Application can only be configured to work with newly created sites/repositories. Before we consider this integration to be "Generally Available," we'll need to provide a mechanism for configuring the application to work with already existing sites/repositories.

In fact, I suppose we'll need to provide guidance for at least 4 permutations for configuring the app.

• Neither GitHub repo nor Pantheon site exist yet (this is what we already have)
• Both the GitHub repo and the Pantheon site exist, and now need to be connected via the app
• The GitHub repo already exists but there is no Pantheon site.
• A Pantheon site exists, but no GitHub repo exists.