Add more events and notifications for tasks

app/Templates/notification_task_assignee_change.php

@@ -0,0 +1,20 @@
+<h2><?= Helper\escape($task['title']) ?> (#<?= $task['id'] ?>)</h2>
+
+<ul>
+    <li>
+        <strong>
+        <?php if ($task['assignee_username']): ?>
+            <?= t('Assigned to %s', $task['assignee_name'] ?: $task['assignee_username']) ?>
+        <?php else: ?>
+            <?= t('There is nobody assigned') ?>
+        <?php endif ?>
+        </strong>
+    </li>
+</ul>
+
+<?php if (! empty($task['description'])): ?>
+    <h2><?= t('Description') ?></h2>
+    <?= Helper\parse($task['description']) ?: t('There is no description.') ?>
+<?php endif ?>
+
+<?= Helper\template('notification_footer', array('task' => $task)) ?>