Bug 1827709: Notify Thunderbird CI Matrix channel after mozilla-central push. r=releng-reviewers,gabriel

This notification will help Thunderbird developers who are tasked with pushing
multiple bugs to comm-central after a mozilla-central push.

Taskgraph tests adjusted to verify the notification is only sent for pushes
to mozilla-central, and to run the tests when .taskcluster.yml is updated.

Differential Revision: https://phabricator.services.mozilla.com/D175290