Add Challenge button in lobby to add max amount of bots for selected map

- use a ~50/50 combination of the two hardest AI
- set them all to team 2
- change LobbyCommands.cs slot_bot to accept a team as parameter
 - {0} = slot, {1} = team, {2} = bot
 - update slot_bot command in LobbyUtils.cs (team param: 0)