Subscribers should be proactively notified by Email if Zoom connection goes stale / expires

Even though we are developing a way to auto-refresh Zoom tokens regularly, there might be cases where this can fail. If it does, it would be nice to proactively notify the subscriber through email!

Related to https://github.com/thunderbird/appointment/issues/1580