[FIX] Bug in material edition
[FIX] Bug in material edition

 - User list would be erased on save;
 - Due to changes in the serialization mechanism;
 - Temporary hack, better solution with upcoming 'fossilize' patch;

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