title: Variable "app" does not exist with Twig
body: <p>I'm trying to get the URL of my page with Twig.
  So I wrote this in my index.html.twig:
  <code>{{ app.request.attributes.get("_route") }}</code></p>
  
  <p>I'm getting this Symfony exception:</p>
  
  <blockquote>
    <p>Variable "app" does not in exist in "AcmeFoo..."</p>
  </blockquote>