Problem: Billing is always on

Solution: Make billing configurable through LIBRETIME_ENABLE_BILLING and deactivate it

This should catch all the changes needed to deactive billing in LibreTime.

* [x] only call billing when it is enabled
* [x] let super admins edit their info
* [x] dont link to billing if it is disabled

airtime_mvc/application/configs/constants.php

@@ -141,3 +141,6 @@ define('STATION_PODCAST_SERVICE_NAME', 'station_podcast');
 //define('IMPORTED_PODCAST', 1);
 
 define('ITUNES_XML_NAMESPACE_URL', 'http://www.itunes.com/dtds/podcast-1.0.dtd');
+
+// Billing configuration
+define('LIBRETIME_ENABLE_BILLING', false);