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/form/editor.php

@@ -377,6 +377,7 @@ class MoodleQuickForm_editor extends HTML_QuickForm_element {
         }
 
         // print text area - TODO: add on-the-fly switching, size configuration, etc.
+        $editor->set_text($text);
         $editor->use_editor($id, $this->_options, $fpoptions);
 
         $rows = empty($this->_attributes['rows']) ? 15 : $this->_attributes['rows'];