Add sorting by reference in list view

app/Model/TaskFinderModel.php

@@ -117,6 +117,7 @@ class TaskFinderModel extends Base
                 TaskModel::TABLE.'.recurrence_child',
                 TaskModel::TABLE.'.time_estimated',
                 TaskModel::TABLE.'.time_spent',
+                TaskModel::TABLE.'.reference',
                 UserModel::TABLE.'.username AS assignee_username',
                 UserModel::TABLE.'.name AS assignee_name',
                 UserModel::TABLE.'.email AS assignee_email',

app/Template/task_list/sort_menu.php

@@ -34,5 +34,8 @@
         <li>
             <?= $paginator->order(t('Status'), \Kanboard\Model\TaskModel::TABLE.'.is_active') ?>
         </li>
+        <li>
+            <?= $paginator->order(t('Reference'), \Kanboard\Model\TaskModel::TABLE.'.reference') ?>
+        </li>
     </ul>
 </div>