Unrank games that are mutual draw

related to #936
Players perceive the button for a draw in the diplomacy menu as "let's pretend this game never happened" and it's primarily used when people don't like the map or there are technical problems like a player losing connection early on. Consequently, we should reintroduce the mutual draw logic that was removed in #940 and fix the issue that everyone had to send MUTUAL_DRAW to unrank, as dropped out players obviously can't send that.