DEV: use upload id to save in theme setting instead of URL. (PR #14341)

when we use URL instead it creates the problem while changing the CDN hostname.


What happens if someone sets the default to the name of a non-upload theme_field (e.g. head)?

I think we need to restrict this lookup to upload theme fields. Maybe by checking that the type is theme_upload_var?

Sure, I will check the type too