[Web] Fix slow UI by switching QR provider and only generating qr image on demand

data/web/inc/ajax/qr_gen.php

@@ -0,0 +1,13 @@
+<?php
+session_start();
+require_once $_SERVER['DOCUMENT_ROOT'] . '/inc/prerequisites.inc.php';
+header('Content-Type: text/plain');
+if (!isset($_SESSION['mailcow_cc_role'])) {
+	exit();
+}
+
+if (isset($_GET['token']) && ctype_alnum($_GET['token'])) {
+  echo $tfa->getQRCodeImageAsDataUri($_SESSION['mailcow_cc_username'], $totp_secret);
+}
+
+?>