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/Presets/PresetDeathMatch.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using Content.Server.GameTicking.Rules;
 using Robust.Server.Player;
+using Robust.Shared.GameObjects;
 using Robust.Shared.IoC;
 
 namespace Content.Server.GameTicking.Presets
@@ -8,11 +9,9 @@ namespace Content.Server.GameTicking.Presets
     [GamePreset("deathmatch")]
     public sealed class PresetDeathMatch : GamePreset
     {
-        [Dependency] private readonly IGameTicker _gameTicker = default!;
-
         public override bool Start(IReadOnlyList<IPlayerSession> readyPlayers, bool force = false)
         {
-            _gameTicker.AddGameRule<RuleDeathMatch>();
+            EntitySystem.Get<GameTicker>().AddGameRule<RuleDeathMatch>();
             return true;
         }