Fix reference number migration
Fix reference number migration

Some legacy objects store a string instead of a list of strings

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