drag-drop: do not allow images to be dropped in a seat
drag-drop: fix bug in dragenter handler

also- make person icon visible only if seat is occupied
pairings: put seat box template into html file
pairings: add menu btn to each seat
pairings(seat): implement 'vacate' action
pairings: only show seat buttons on occupied seats
pairings: when adding a player, fill empty seats before creating new seats