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
To be consistent with other forms, let's change this to:
```php
$form['actions']['#type'] = 'actions';
$form['actions']['submit'] = array(
  '#type' => 'submit',
  '#value' => t('Save configuration'),
);
```