title: Callback URL error when setting up webhook for messenger
body: <p>I'm trying to follow this <a href="https://blog.hartleybrody.com/fb-messenger-bot/" rel="nofollow">tutorial</a> to set a chatbot for messenger. I'm stuck on the webhook setup. I added the page token and verify token to heroku, but when I try to add the heroku URL as the callback URL I get </p>
  
  <blockquote>
    <p>The URL couldn't be validated. Callback verification failed with the
    following errors: HTTP Status Code = 403; HTTP Message = FORBIDDEN</p>
  </blockquote>