callvote lock, unlock and an option to start games with locked teams

[Irbyz]

callvote lock and callvote unlock could be added as custom votes like suggested in #28, however these commands will be probably in high demand so they could be implemented on their own.
Together with an option to start games locked automatically, after players did ready up.

Then there could be some way to make it easier for spectators to join mid game. Maybe by letting spectators to callvote unlock and/or somehow making sure that there are two players ready to join and then letting them join, for example a player who wants to join could be placed in a queue to join, and once there's another who joins the queue with them, they would both end up joining teams, even if teams are locked.