title: Get absolute or relative url of wagtail page
body: <p>I have to following code snippet which should return a correct url (relative or absolute)</p>
  
  <pre><code>class LinkFields(models.Model):
      link_external   = models.URLField("External link", blank=True)
      link_page       = models.ForeignKey('wagtailcore.Page', null=True, blank=True, related_name='+')
      link_document   = models.ForeignKey('wagtaildocs.Document', null=True, blank=True, related_name='+' )
  
      @property
      def url(self):
          if self.link_page:
              return self.link_page.url
          elif self.link_document:
              return self.link_document.url
          else:
              return self.link_external
  
      panels = [
          FieldPanel('link_external'),
          PageChooserPanel('link_page'),
          DocumentChooserPanel('link_document'),
      ]
  
      class Meta:
          abstract = True
  </code></pre>
  
  <p>So if I use the ".url" property for a "wagtailcore.Page" a get an absolute url starting with "http" instead of "https".</p>
  
  <p>What is the correct wagtail way to return a correct relative url or correct absolute url (leading with https in my case) within  my "view/model"?</p>
  
  <p>Thank you</p>