Support GitHub App on an App level

[pjlast]

Our current GitHub App code host connections require an Installation ID, and is tied to a single App installation.

This has a limitation in that a new code host connection needs to be added for each installation. However, in setups where someone might have different orgs within their companies that get created and deleted, it could be desirable to have a single GitHub App code host connection that is not tied to a single installation, and instead works with all orgs the app is installed on.

This should be entirely possible, since we have access to the AppID, so we can use this endpoint to fetch a list of installations and iterate over them and sync repositories.

Some issues we may run into:

• I think we'll need to store the installation ID of each installation associated with a repository. Otherwise I'm not sure how we'd do repo updates when we try to fetch a repo again. We'll need to grab an installation access token to do all of that, so we'll need the corresponding installation ID.
• The GitHub code host connections code is going to get very messy, because we now have branching paths for PATs, Apps with Installation IDs, and Apps only. I'd recommend some heavy refactoring there to split these out into different code paths, instead of complicating the if statements even further.
  • We have tons of places in the GitHub code host connections where we do checks like this, which is where the additional check would be. I think we need to decide which code host connection it is when the connection gets created, and greatly simplify these checks.