Subtasks events refactoring and show delete in activity stream

app/Template/event/subtask_delete.php

@@ -0,0 +1,15 @@
+<p class="activity-title">
+    <?= e('%s removed a subtask for the task %s',
+        $this->text->e($author),
+        $this->url->link(t('#%d', $task['id']), 'TaskViewController', 'show', array('task_id' => $task['id'], 'project_id' => $task['project_id']))
+    ) ?>
+    <span class="activity-date"><?= $this->dt->datetime($date_creation) ?></span>
+</p>
+<div class="activity-description">
+    <p class="activity-task-title"><?= $this->text->e($task['title']) ?></p>
+    <ul>
+        <li>
+            <?= $this->text->e($subtask['title']) ?> (<strong><?= $this->text->e($subtask['status_name']) ?></strong>)
+        </li>
+    </ul>
+</div>