mark POSTPONED or CANCELED Indico events with a nice flag and notifications where appropriate

[mariadimou]

When an Indico event is POSTPONED or CANCELLED, it would be nice to provide a relevant flag, rather than the event manager putting vulgar upper-case letters in the title. Example:
https://indico.cern.ch/event/673053/

Also to generate notifications where appropriate, e.g. the audiovisual team, if recording were planned and the registered participants, when registration is involved.

I suggested it to Pedro who said this is a good enhancement. Thanks for your attention.