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/Players/PlayerSessionExt.cs

@@ -1,14 +0,0 @@
-using Content.Shared.GameWindow;
-using Robust.Server.Player;
-
-namespace Content.Server.Players
-{
-    public static class PlayerSessionExt
-    {
-        public static void RequestWindowAttention(this IPlayerSession session)
-        {
-            var msg = session.ConnectedClient.CreateNetMessage<MsgRequestWindowAttention>();
-            session.ConnectedClient.SendMessage(msg);
-        }
-    }
-}