This change adds the connection flow and UI for using GitHub Apps. This

involves a new set of views that deal with the mechanics of triggering

the app creation, processing the result, and triggering an app install.
This also includes a BaseHostingServiceConnectUI subclass for GitHub

that wires all of that into the connect wizard and provides some custom

UI for the GitHub entry in the connected services list.

Connected a GitHub App and installed it into multiple organizations.
Tested all aspects of the wizard flow, including creation of the app

  and reassignment of any PAT-based repositories after app install.
Verified that API access works correctly when authenticating using the

  app tokens.
Ran unit tests.