Port to Backdrop CMS

Thanks to the creator of this module and to the people who submitted patches for additional functions. I've ported this module to Backdrop CMS at https://backdropcms.org/project/url_unpublish / https://github.com/backdrop-contrib/url_unpublish

I've also included the functionality to include a publish URL provided by https://www.drupal.org/project/url_unpublish/issues/1953940 → and the Views Handler at https://www.drupal.org/project/url_unpublish/issues/1691754 →