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/Administration/Commands/AGhost.cs

@@ -42,7 +42,7 @@ namespace Content.Server.Administration.Commands
             var canReturn = mind.CurrentEntity != null;
             var ghost = IoCManager.Resolve<IEntityManager>()
                 .SpawnEntity("AdminObserver", player.AttachedEntity?.Transform.Coordinates
-                                              ?? IoCManager.Resolve<IGameTicker>().GetObserverSpawnPoint());
+                                              ?? EntitySystem.Get<GameTicker>().GetObserverSpawnPoint());
 
             if (canReturn)
             {