Do not show participation acc/rej form if sent
Do not show participation acc/rej form if sent

Once the user has accepted/refused the participation invitation
showing the form again resulted in a not very helpful error message.
Instead we now tell the user that he already accepted/rejected the
invitation.

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