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/Spawners/EntitySystems/ConditionalSpawnerSystem.cs

@@ -0,0 +1,26 @@
+using Content.Server.GameTicking;
+using Content.Server.Spawners.Components;
+using JetBrains.Annotations;
+using Robust.Shared.GameObjects;
+
+namespace Content.Server.Spawners.EntitySystems
+{
+    [UsedImplicitly]
+    public class ConditionalSpawnerSystem : EntitySystem
+    {
+        public override void Initialize()
+        {
+            base.Initialize();
+
+            SubscribeLocalEvent<GameRuleAddedEvent>(OnRuleAdded);
+        }
+
+        private void OnRuleAdded(GameRuleAddedEvent args)
+        {
+            foreach (var spawner in ComponentManager.EntityQuery<ConditionalSpawnerComponent>())
+            {
+                spawner.RuleAdded(args);
+            }
+        }
+    }
+}