Add email notifications

app/Templates/notification_task_due.php

@@ -0,0 +1,10 @@
+<h2><?= t('List of due tasks for the project "%s"', $project) ?></h2>
+
+<ul>
+    <?php foreach ($tasks as $task): ?>
+        <li>(<strong>#<?= $task['id'] ?></strong>) <?= Helper\escape($task['title']) ?> (<strong><?= t('Assigned to %s', $task['assignee_name'] ?: $task['assignee_username']) ?></strong>)</li>
+    <?php endforeach ?>
+</ul>
+
+<hr/>
+<p>Kanboard</p>