Issue #1493098 by sun, jcisio, pcambra, Pol, wamilton, Rok Žlender, beejeebus: convert cron settings to configuration system.

PR for https://github.com/backdrop/backdrop-issues/issues/170
@quicksketch I'm not sure if it's necessary, for this function I was just following [this patch](https://www.drupal.org/files/drupal8.config-cron.76.patch). I'll try taking it out and see if it works.