Disallow merging admin into non-admin
Disallow merging admin into non-admin

fixes #1894

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