Fix submission of EventPersonListField
Fix submission of EventPersonListField

It was previously impossible to create a contribution after a validation
error because we tried to use an EventPerson which had not been
persisted to the database.

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