MDL-57596 forms: CLEANHTML in persistent forms

Add special handling for text fields with the CLEANHTML type. This should
be used when students and teachers can edit the same field (you can't trust those students).

Applies cleaning on submitted data, and on data stored in the DB before it is put back in an editing form.

lib/classes/form/persistent.php

@@ -223,9 +223,14 @@ abstract class persistent extends moodleform {
         $data = $this->get_persistent()->to_record();
         $class = static::$persistentclass;
         $properties = $class::get_formatted_properties();
+        $allproperties = $class::properties_definition();
 
         foreach ($data as $field => $value) {
-            // Convert formatted properties.
+            // Clean data if it is to be displayed in a form.
+            if (isset($allproperties[$field]['type'])) {
+                $data->$field = clean_param($data->$field, $allproperties[$field]['type']);
+            }
+
             if (isset($properties[$field])) {
                 $data->$field = array(
                     'text' => $data->$field,