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/GameTicker.GameRule.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+using Content.Server.GameTicking.Rules;
+using Robust.Shared.ViewVariables;
+
+namespace Content.Server.GameTicking
+{
+    public partial class GameTicker
+    {
+        [ViewVariables] private readonly List<GameRule> _gameRules = new();
+        public IEnumerable<GameRule> ActiveGameRules => _gameRules;
+
+        public T AddGameRule<T>() where T : GameRule, new()
+        {
+            var instance = _dynamicTypeFactory.CreateInstance<T>();
+
+            _gameRules.Add(instance);
+            instance.Added();
+
+            RaiseLocalEvent(new GameRuleAddedEvent(instance));
+
+            return instance;
+        }
+
+        public bool HasGameRule(string? name)
+        {
+            if (name == null)
+                return false;
+
+            foreach (var rule in _gameRules)
+            {
+                if (rule.GetType().Name == name)
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        public bool HasGameRule(Type? type)
+        {
+            if (type == null || !typeof(GameRule).IsAssignableFrom(type))
+                return false;
+
+            foreach (var rule in _gameRules)
+            {
+                if (rule.GetType().IsAssignableFrom(type))
+                    return true;
+            }
+
+            return false;
+        }
+
+        public void RemoveGameRule(GameRule rule)
+        {
+            if (_gameRules.Contains(rule)) return;
+
+            rule.Removed();
+
+            _gameRules.Remove(rule);
+        }
+    }
+
+    public class GameRuleAddedEvent
+    {
+        public GameRule Rule { get; }
+
+        public GameRuleAddedEvent(GameRule rule)
+        {
+            Rule = rule;
+        }
+    }
+}