Add helper for easy migration from the wmsettings module

This turned out to be pretty simple since only config migration is required, no data. The entity type and bundles stay the same.

For future reference: the easiest thing to do here is to enable eck_site_settings, deploy to all environments and only then migrate and uninstall wmsettings. Also, don't forget any code changes like usage of the WmSettings service and the setting Twig extension.

Automatically closed - issue fixed for 2 weeks with no activity.