Find the full URL for a help token.

def url_for_token(self, token):
        """Find the full URL for a help token."""
        book_url = self.get_config_value("pages", token)
        book, _, url_tail = book_url.partition(':')
        book_base = settings.HELP_TOKENS_BOOKS[book]

        url = book_base

        lang = getattr(settings, "HELP_TOKENS_LANGUAGE_CODE", None)
        if lang is not None:
            lang = self.get_config_value("locales", lang)
            url += "/" + lang

        version = getattr(settings, "HELP_TOKENS_VERSION", None)
        if version is not None:
            url += "/" + version

        url += "/" + url_tail
        return url