title: Wagtail says "Page not found" when trying to create a new page in Wagtail admin
body: <p>I am getting the following error when I try to create a new page in Wagtail admin:</p>
  
  <pre><code>Page not found (404)
  Request Method: GET
  Request URL:    http://127.0.0.1:8000/wagtail-admin/pages/add/blog/homepage/1/
  Raised by:  wagtail.wagtailadmin.views.pages.create
  You're seeing this error because you have DEBUG = True in your Django settings file. Change that to False, and Django will display a standard 404 page.
  </code></pre>
  
  <p>I have added the new page model to <code>models.py</code> like this:</p>
  
  <pre><code>...
  class HomePage(Page):
      pass
  ...
  </code></pre>
  
  <p>What am I doing wrong?</p>