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/AI/Components/AiControllerComponent.cs

@@ -46,7 +46,7 @@ namespace Content.Server.AI.Components
 
             if (StartingGearPrototype != null)
             {
-                var gameTicker = IoCManager.Resolve<IGameTicker>();
+                var gameTicker = EntitySystem.Get<GameTicker>();
                 var protoManager = IoCManager.Resolve<IPrototypeManager>();
 
                 var startingGear = protoManager.Index<StartingGearPrototype>(StartingGearPrototype);