Migrate push notifications from post-receive hook on cgit to GitLab

See https://docs.gitlab.com/ee/user/project/integrations/emails_on_push.html

We would configure that per repo and put the GStreamer commit mailing list in there as receiver.

Any opinions on this? CC @daniels @bilboed @tpm?

• emails sent to commit ml on every push from gitlab
• Make sure legacy post-commit hooks are disabled @tpm ? @daniels ?