Save the old slug to be used later in PageAdmin.save_model()
        to make the slug change propagate down the page tree, and clean
        leading and trailing slashes which are added on elsewhere.

def clean_slug(self):
        """
        Save the old slug to be used later in PageAdmin.save_model()
        to make the slug change propagate down the page tree, and clean
        leading and trailing slashes which are added on elsewhere.
        """
        self.instance._old_slug = self.instance.slug
        new_slug = self.cleaned_data['slug']
        if not isinstance(self.instance, Link) and new_slug != "/":
            new_slug = clean_slashes(self.cleaned_data['slug'])
        return new_slug