Adding PWA support.

src/tpl/footer.php

@@ -9,5 +9,13 @@ foreach ($js_modules as $mod) {
 }
 ?>
 </script>
+
+<script>
+if ('serviceWorker' in navigator) {
+  window.addEventListener('load', () => {
+    navigator.serviceWorker.register('/sw.js').catch(() => {});
+  });
+}
+</script>
 </body>
 </html>