Turns GameTicker into an EntitySystem. (#4197)

* GameTicker turned into an EntitySystem

* Turns ClientGameTicker into an EntitySystem, turn NetMessages into events

* Change event names to be more consistent with the rest.

* YAML linter uses the dummy gameticker CVar override.

* Fix game ticker initialization order

* Dummy ticker won't spawn players.

* Fix character creation test

Content.Server/GameTicking/Commands/RespawnCommand.cs

@@ -1,6 +1,7 @@
 using Content.Server.Players;
 using Robust.Server.Player;
 using Robust.Shared.Console;
+using Robust.Shared.GameObjects;
 using Robust.Shared.IoC;
 using Robust.Shared.Network;
 
@@ -22,7 +23,7 @@ namespace Content.Server.GameTicking.Commands
             }
 
             var playerMgr = IoCManager.Resolve<IPlayerManager>();
-            var ticker = IoCManager.Resolve<IGameTicker>();
+            var ticker = EntitySystem.Get<GameTicker>();
 
             NetUserId userId;
             if (args.Length == 0)