Handle GitHub App revoked credentials webhook

Created by: pjlast

We should proactively handle GitHub App revocations using the mandatory webhook, as mentioned here: https://docs.github.com/en/developers/apps/building-github-apps/identifying-and-authorizing-users-for-github-apps#handling-a-revoked-github-app-authorization

When this webhook is received, the user's corresponding GitHub App entries in user_external_services should be removed (or marked deleted, either should be fine).