MDL-55417 forms: Render form elements with a template

This change allows form elements to be overridden with a mustache template.
The template can even listen for form validation errors and supply the JS to
change the look of the form element when there is/isn't a validation error.

Initial support is for all core form elements including:

text, select, selectyesno and checkboxes, groups, dateselector, datetimeselector,
autocomplete, modvisible, advcheckbox, button, duration, filemanager, filepicker, editor, static, grading,
warning, textarea, password, url, submit, questioncategory, recaptcha.

Part of MDL-55071

lib/outputrenderers.php

@@ -4421,6 +4421,51 @@ EOD;
         return $this->render_from_template('core/login', $context);
     }
 
+    /**
+     * Renders an mform element from a template.
+     *
+     * @param HTML_QuickForm_element $element element
+     * @param bool $required if input is required field
+     * @param string $error error message to display
+     * @param bool $ingroup True if this element is rendered as part of a group
+     * @return mixed string|bool
+     */
+    public function mform_element($element, $required, $error, $ingroup) {
+        $templatename = 'core_form/element-' . $element->getType();
+        if ($ingroup) {
+            $templatename .= "-inline";
+        }
+        try {
+            // We call this to generate a file not found exception if there is no template.
+            // We don't want to call export_for_template if there is no template.
+            core\output\mustache_template_finder::get_template_filepath($templatename);
+
+            if ($element instanceof templatable) {
+                $elementcontext = $element->export_for_template($this);
+
+                $helpbutton = '';
+                if (method_exists($element, 'getHelpButton')) {
+                    $helpbutton = $element->getHelpButton();
+                }
+                $label = $element->getLabel();
+                if (method_exists($element, 'getText')) {
+                    $label .= ' ' . $element->getText();
+                }
+
+                $context = array(
+                    'element' => $elementcontext,
+                    'label' => $label,
+                    'required' => $required,
+                    'helpbutton' => $helpbutton,
+                    'error' => $error
+                );
+                return $this->render_from_template($templatename, $context);
+            }
+        } catch (Exception $e) {
+            // No template for this element.
+            return false;
+        }
+    }
 }
 
 /**