Allow game presets to disallow latejoining (#1816)

* Allow game presets to disallow latejoining

* Update Content.Server/GameTicking/GameTicker.cs

Co-authored-by: DrSmugleaf <DrSmugleaf@users.noreply.github.com>

Co-authored-by: DrSmugleaf <DrSmugleaf@users.noreply.github.com>

Content.Server/GameTicking/GamePreset.cs

@@ -12,6 +12,7 @@ namespace Content.Server.GameTicking
         public abstract bool Start(IReadOnlyList<IPlayerSession> readyPlayers, bool force = false);
         public virtual string ModeTitle => "Sandbox";
         public virtual string Description => "Secret!";
+        public virtual bool DisallowLateJoin => false;
         public Dictionary<string, HumanoidCharacterProfile> readyProfiles;
     }
 }