CfA: add email notification for track changes

[micsucmed]

IPAC Scientific Programme Committee highlighted a limitation in the process of abstracts review.
Whenever one MC Coordinator (= track convener) moved one abstract to another track, the destination track convener missed that abstract because no evidence of the moval is available in Indico.

They proposed that any track convener "receiving" an abstract is notified via email.

This makes mush sense to me so I'm proposing this as a feature request.

I believe the track change is the more important one to get notifications for.
However, it could be expanded adding notifications to conveners for any action by reviewers (propose to XXX).

In that case, it would be useful to have a panel where the administrator can enable/disable every single action, if the committee only wants to filter some actions out.