Authenticates/installs this app for the current user.

def authenticate_redirect(self, callback_uri=None, cancel_uri=None,
                              extended_permissions=None):
        """Authenticates/installs this app for the current user."""
        self.require_setting("facebook_api_key", "Facebook Connect")
        callback_uri = callback_uri or self.request.uri
        args = {
            "api_key": self.settings["facebook_api_key"],
            "v": "1.0",
            "fbconnect": "true",
            "display": "page",
            "next": urljoin(self.request.full_url(), callback_uri),
            "return_session": "true",
        }
        if cancel_uri:
            args["cancel_url"] = urljoin(
                self.request.full_url(), cancel_uri)
        if extended_permissions:
            if isinstance(extended_permissions, (str, bytes_type)):
                extended_permissions = [extended_permissions]
            args["req_perms"] = ",".join(extended_permissions)
        self.redirect("http://www.facebook.com/login.php?" +
                      urlencode(args))