otp: show secret as plaintext if a user can't scan qrcode

Fixes #4818

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

src/Wallabag/CoreBundle/Controller/ConfigController.php

@@ -344,6 +344,7 @@ class ConfigController extends Controller
         return $this->render('WallabagCoreBundle:Config:otp_app.html.twig', [
             'backupCodes' => $backupCodes,
             'qr_code' => $this->get('scheb_two_factor.security.google_authenticator')->getQRContent($user),
+            'secret' => $secret,
         ]);
     }