Allow editing of site URL after product is in the cart

When you add a site product to the cart, you get redirected to a pane where you can enter the site URL.

However, if you leave that page, you're stuck with a site product with an empty URL. Also, if you make a typo in the URL, you have to remove and re-add a product, which is a bit unintuitive.

I think it would also allow to actually implement "skip this step" in the site URL pane (which, as far as I can tell, does nothing right now).