Investigate migrating away from classic PATs in GH sync

Github is pushing more towards fine-grained tokens over classic access tokens, along with other options that are more secure with shorter-lived tokens. One other way that we could accomplish this would be to move to a Github App instead of using an owner's personal access token. This would give us short-lived tokens and would help resolve the rate-limiting issues in the sync operation.

Sol'n	Pros	Cons
Fine-grained Token	Easier to implement, next to no code	Needs manual intervention to renew the token, needs validation that all endpoints are supported
Github App	Alleviates issues with rate limits, no renewal period for token	Significantly work to implement

@cguindon any preference on this? I threw in the GH App as that would functionally do the same thing as we'd no longer be using a PAT and we'd be able to access resources more naturally through installations, and there are plans in the work to do it eventually.