Subtasks events refactoring and show delete in activity stream

app/Template/notification/subtask_delete.php

@@ -0,0 +1,11 @@
+<h2><?= $this->text->e($task['title']) ?> (#<?= $task['id'] ?>)</h2>
+
+<h3><?= t('Subtask removed') ?></h3>
+
+<ul>
+    <li><?= t('Title:') ?> <?= $this->text->e($subtask['title']) ?></li>
+    <li><?= t('Status:') ?> <?= $this->text->e($subtask['status_name']) ?></li>
+    <li><?= t('Assignee:') ?> <?= $this->text->e($subtask['name'] ?: $subtask['username'] ?: '?') ?></li>
+</ul>
+
+<?= $this->render('notification/footer', array('task' => $task, 'application_url' => $application_url)) ?>