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
Do we need all this rearrangement in `backdrop_install_system()`? I don't think installing the cron settings should affect this function (but maybe it does?)