Use Frontend extender for embedding statistics in admin frontend

Refs flarum/core#1602, flarum/core#1608.

extensions/statistics/extend.php

@@ -11,14 +11,10 @@
 
 use Flarum\Extend;
 use Flarum\Statistics\Listener;
-use Illuminate\Contracts\Events\Dispatcher;
 
 return [
     (new Extend\Frontend('admin'))
         ->js(__DIR__.'/js/dist/admin.js')
-        ->css(__DIR__.'/less/admin.less'),
-
-    function (Dispatcher $events) {
-        $events->subscribe(Listener\AddStatisticsData::class);
-    }
+        ->css(__DIR__.'/less/admin.less')
+        ->content(Listener\AddStatisticsData::class),
 ];