Add email notifications

app/Templates/notification_subtask_update.php

@@ -0,0 +1,22 @@
+<h2><?= Helper\escape($task['title']) ?> (#<?= $task['id'] ?>)</h2>
+
+<h3><?= t('Sub-task updated') ?></h3>
+
+<ul>
+    <li><?= t('Title:') ?> <?= Helper\escape($subtask['title']) ?></li>
+    <li><?= t('Status:') ?> <?= Helper\escape($subtask['status_name']) ?></li>
+    <li><?= t('Assignee:') ?> <?= Helper\escape($subtask['name'] ?: $subtask['username'] ?: '?') ?></li>
+    <li>
+        <?= t('Time tracking:') ?>
+        <?php if (! empty($subtask['time_spent'])): ?>
+            <strong><?= Helper\escape($subtask['time_spent']).'h' ?></strong> <?= t('spent') ?>
+        <?php endif ?>
+
+        <?php if (! empty($subtask['time_estimated'])): ?>
+            <strong><?= Helper\escape($subtask['time_estimated']).'h' ?></strong> <?= t('estimated') ?>
+        <?php endif ?>
+    </li>
+</ul>
+
+<hr/>
+<p>Kanboard</p>