MDL-66118 hub: Define the new sites directory URL

The Moodle sites now register with stats.moodle.org so use that as a new
official hub's URL. Beside that, allow for the alternative value be
provided in config.php for development and testing purposes.

lib/moodlelib.php

@@ -487,9 +487,9 @@ define('HOMEPAGE_MY', 1);
 define('HOMEPAGE_USER', 2);
 
 /**
- * Moodle.net url (should be moodle.net)
+ * URL of the Moodle sites registration portal.
  */
-define('HUB_MOODLEORGHUBURL', "https://moodle.net");
+defined('HUB_MOODLEORGHUBURL') || define('HUB_MOODLEORGHUBURL', 'https://stats.moodle.org');
 
 /**
  * Moodle mobile app service name