Fix join and docstring of user_registered_in_event
Fix join and docstring of user_registered_in_event

- indentation
- event is not an Event object (but legacy Conference)
- also use the common indico style to wrap the query
- use relationship to join instead of explicitly specifying the criteria

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