MDL-61514 Quiz: Display unavailable tags when editing random questions

When editing a random question, the form will display the tags that the random question
is configured to get questions from even if those tags do not exist anymore.

mod/quiz/classes/form/randomquestion_form.php

@@ -81,4 +81,20 @@ class randomquestion_form extends \moodleform {
         $mform->addGroup($buttonarray, 'buttonar', '', array(' '), false);
         $mform->closeHeaderBefore('buttonar');
     }
+
+    public function set_data($defaultvalues) {
+        $mform = $this->_form;
+
+        if ($defaultvalues->fromtags) {
+            $fromtagselement = $mform->getElement('fromtags');
+            foreach ($defaultvalues->fromtags as $fromtag) {
+                if (!$fromtagselement->optionExists($fromtag)) {
+                    $optionname = get_string('randomfromunavailabletag', 'mod_quiz', explode(',', $fromtag)[1]);
+                    $fromtagselement->addOption($optionname, $fromtag);
+                }
+            }
+        }
+
+        parent::set_data($defaultvalues);
+    }
 }

mod/quiz/lang/en/quiz.php

@@ -700,6 +700,7 @@ $string['randomcreate'] = 'Create random questions';
 $string['randomediting'] = 'Editing a random question';
 $string['randomfromcategory'] = 'Random question from category:';
 $string['randomfromexistingcategory'] = 'Random question from an existing category';
+$string['randomfromunavailabletag'] = '{$a} (unavailable)';
 $string['randomnumber'] = 'Number of random questions';
 $string['randomnosubcat'] = 'Questions from this category only, not its subcategories.';
 $string['randomquestion'] = 'Random question';