Use urljoin in Page.get_absolute_url for link pages

I came upon this when I wanted to add a `mailto:` url as a link. `urljoin` from Python's standard library handles more schemes and edge cases. Another way to add a few schemes could be:

    url_schemes = ('http:', 'https:', 'mailto:', 'ftp:')
    if not slug.lower().startswith(url_schemes):
        slug = "/" + self.slug.lstrip("/")

If you don't like `urljoin`, I can send a pull request with this fix.