Save php sessions in myaac dir

Instead of default PHP location
This fixes problem with permissions

common.php

@@ -24,7 +24,6 @@
  * @link      https://my-aac.org
  */
 if (version_compare(phpversion(), '7.1', '<')) die('PHP version 7.1 or higher is required.');
-session_start();
 
 const MYAAC = true;
 const MYAAC_VERSION = '0.9.0-dev';
@@ -87,6 +86,9 @@ const TFS_03 = 4;
 const TFS_FIRST = TFS_02;
 const TFS_LAST = TFS_03;
 
+session_save_path(SYSTEM . 'php_sessions');
+session_start();
+
 // basedir
 $basedir = '';
 $tmp = explode('/', $_SERVER['SCRIPT_NAME']);