add 'lock teams' feature backend -- still needs UI

OpenRA.FileFormats/Session.cs

@@ -53,6 +53,7 @@ namespace OpenRA.FileFormats
 			public string[] Mods = { "ra" };	// mod names
 			public int OrderLatency = 3;
 			public int RandomSeed = 0;
+			public bool LockTeams = false;	// don't allow team changes after game start.
 		}
 	}
 

OpenRA.Game/Widgets/Delegates/DiplomacyDelegate.cs

@@ -116,6 +116,9 @@ namespace OpenRA.Widgets.Delegates
 
 		void CycleStance(Player p, ButtonWidget bw)
 		{
+			if (Game.LobbyInfo.GlobalSettings.LockTeams)
+				return;	// team changes are banned
+
 			var nextStance = GetNextStance((Stance)Enum.Parse(typeof(Stance), bw.Text));
 
 			Game.IssueOrder(new Order("SetStance", Game.world.LocalPlayer.PlayerActor,