Show rules on first connection (#5715)

* Implement showing rules on first connection

* Clean up RulesManager

* Change changelog and rules unread to use CVars

* Fix missing change

* Rename InfoWindow to RulesAndInfoWindow

* Change default server.id to unknown

* Fix invalid file contents and getting CVar

Content.Shared/CCVar/CCVars.cs

@@ -7,6 +7,16 @@ namespace Content.Shared.CCVar
     [CVarDefs]
     public sealed class CCVars : CVars
     {
+        /*
+         * Server
+         */
+
+        /// <summary>
+        ///     Change this to have the changelog and rules "last seen" date stored separately.
+        /// </summary>
+        public static readonly CVarDef<string> ServerId =
+            CVarDef.Create("server.id", "unknown_server_id");
+
         /*
          * Ambience
          */