Change ApiContentUpdateForDemoController to save from auto-save instead of request data

The only drawback of this method is that "Publish" will not work with a complete empty canvas. But since this is temporary button I think that is fine.

This looks like a nice cleanup to me

Thanks @larowlan and @effulgentsia

While reviewing this, I found 📌 LayoutPreview schema should have required properties Active . Would be nice to tighten that up.