Disable Site Design sidebar in CoBlocks Labs if Go function is not defined.

[Full Site Editing](https://github.com/godaddy-wordpress/go/pull/855) work is currently underway in Go and with that the [Site Design feature enabled through CoBlocks Labs will be defunct](https://github.com/godaddy-wordpress/go/issues/856). We need to prevent any attempt to load this feature and it's assets if we detect that the `Go\Core\get_design_style()` function is not defined.