Added a 'leftmargin' field

Also renamed the 'margin' field to 'rightmargin'
to easily distinguish between the two.

backup/moodle2/backup_customcert_stepslib.php

@@ -49,7 +49,7 @@ class backup_customcert_activity_structure_step extends backup_activity_structur
         // The pages.
         $pages = new backup_nested_element('pages');
         $page = new backup_nested_element('page', array('id'), array(
-            'customcertid', 'width', 'height', 'margin',
+            'customcertid', 'width', 'height', 'leftmargin', 'rightmargin',
             'pagenumber', 'timecreated', 'timemodified'));
 
         // The elements.