MDL-51179 Atto: Extend autosave fix to cover text changes

Added new functions to editor api - set/get_text so the
original form text can be determined from an editor.

When calling use_editor() you should first call set_text() with
the text that will be inserted in the form element.

There is also a new scheduled task for cleaning Atto autosave drafts.

lib/adminlib.php

@@ -2319,6 +2319,7 @@ class admin_setting_confightmleditor extends admin_setting_configtext {
         }
 
         $editor = editors_get_preferred_editor(FORMAT_HTML);
+        $editor->set_text($data);
         $editor->use_editor($this->get_id(), array('noclean'=>true));
 
         return format_admin_setting($this, $this->visiblename,