title: Custom URL for Wagtail model
body: <p>I have a model like this:</p>
  
  <pre><code>from wagtail.wagtailcore.models import Page
  
  class Blog(Page):
      created = models.DateTimeField(auto_now_add=True)
      ...
      ..
  </code></pre>
  
  <p>Right Now, my default, if my slug is <code>hi-there</code>, the blog post is accessible on <code>site_url:/hi-there/</code> but I want it accessible via <code>site:url/2014/02/05/hi-there/</code> The page has various methods like <code>url, url_path</code> which one should I override and what's the best practice to achieve something like this in wagtail?</p>