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. Testing Done: - 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.