Define the default moderator group ID

This allows extensions to add default permissions for moderators,
without having to hardcode in the default moderator group ID.

src/Core/Group.php

@@ -52,6 +52,11 @@ class Group extends AbstractModel
      */
     const MEMBER_ID = 3;
 
+    /**
+     * The ID of the mod group.
+     */
+    const MODERATOR_ID = 4;
+
     /**
      * Boot the model.
      *