Send notifications on user mentions

app/Template/notification/task_user_mention.php

@@ -0,0 +1,7 @@
+<h2><?= t('You were mentioned in the task #%d', $task['id']) ?></h2>
+<p><?= $this->e($task['title']) ?></p>
+
+<h2><?= t('Description') ?></h2>
+<?= $this->text->markdown($task['description']) ?>
+
+<?= $this->render('notification/footer', array('task' => $task, 'application_url' => $application_url)) ?>