GitHub App as a possible code host in site admin

Created by: pjlast

Site admins need to be able to configure GitHub App as a site-level code host. Current configs exist, but would have to be updated to remove dotcom and cloud references. These are in the Site configuration section of Site Admin.

Currently the setup allows for a single GitHub App to exist on an instance, because that is how Cloud would have operated. I think it would theoretically be possible to accommodate for multiple GitHub Apps with some clever management of the state variable, but I don't think this is how it's intended to be used, so I'd rather not explore that route.

I think it would be great if there was a GitHub App entry under Manage code hosts, which takes the admin to setup instructions on how to create the app and which configurations to add to the Site configuration