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.IntegrationTests/Tests/ResettingEntitySystemTests.cs

@@ -36,8 +36,8 @@ namespace Content.IntegrationTests.Tests
 
             await server.WaitIdleAsync();
 
-            var gameTicker = server.ResolveDependency<IGameTicker>();
             var entitySystemManager = server.ResolveDependency<IEntitySystemManager>();
+            var gameTicker = entitySystemManager.GetEntitySystem<GameTicker>();
 
             await server.WaitAssertion(() =>
             {