Prevent New Players from Joining Ranked Matches in Progress

Prevent matchmaking from placing new players into ranked matches that have already started. Players can currently join near the end of a ranked match and lose ELO for a result they had little opportunity to influence, particularly when joining a team that is already at a disadvantage. Ranked matches should begin with complete teams and remain closed to new participants after starting. Players originally assigned to the match should still be allowed to reconnect following a disconnection.