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/form/submitlink.php

@@ -20,6 +20,7 @@
  *
  * Contains HTML class for a submitting to link
  *
+ * @deprecated since 3.2
  * @package   core_form
  * @copyright 2006 Jamie Pratt <[email protected]>
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
@@ -52,6 +53,7 @@ class MoodleQuickForm_submitlink extends MoodleQuickForm_submit {
      * @param string $attributes (optional) Either a typical HTML attribute string or an associative array
      */
     public function __construct($elementName=null, $value=null, $attributes=null) {
+        debugging('Element type submitlink is deprecated.', DEBUG_DEVELOPER);
         parent::__construct($elementName, $value, $attributes);
     }