Only log event ACL change if something changed
Only log event ACL change if something changed

Otherwise each chairperson change spams the log because of the
update_principal call for submission rights

DirkHoffmann/indico,ThiefMaster/indico,mvidalgarcia/indico,ThiefMaster/indico,mic4ael/indico,indico/indico,indico/indico,ThiefMaster/indico,mvidalgarcia/indico,OmeGak/indico,mvidalgarcia/indico,pferreir/indico,indico/indico,pferreir/indico,mvidalgarcia/indico,OmeGak/indico,DirkHoffmann/indico,mic4ael/indico,OmeGak/indico,pferreir/indico,OmeGak/indico,mic4ael/indico,mic4ael/indico,pferreir/indico,DirkHoffmann/indico,ThiefMaster/indico,indico/indico,DirkHoffmann/indico