a8n: Upsert changeset events from GitHub webhooks
Created by: tsenart
This PR introduces a GitHub organization webhook handler that converts relevant incoming events for @sourcegraph/automation into a ChangesetEvent
and upserts it into Postgres.
The documentation has been updated for the additional webhooks
configuration setting in GitHub external services in a way the lets us use webhooks for more than a8n in the future (e.g. for repo metadata).