title: Wagtail: change max_length of Page.title
body: <p>I'm working on a website for a public administration. In this website some pages might have a title which is longer than 255 characters. </p>
  
  <p><strong>Is it possible to change the default max_length of the Page title?</strong></p>
  
  <p>Thanks for the help</p>
  
  <p>I'm using:</p>
  
  <ul>
  <li>Python 2.7;</li>
  <li>Django 1.11;</li>
  <li>Wagtail 1.13.4;</li>
  </ul>