Webhook events are not registered properly for GitHub PRs

On GitHub PR updates we are missing proper `pull_request.synchronize` handling for webhook event registration. 

Also, it we're triggering two prebuilds for the same commit, one for the PR event (context URL is the PR,) and another for the `push` event (context URL is the branch.)

Follow-up to https://github.com/gitpod-io/gitpod/pull/11356