Add support for previewing the order-confirmation page

[mridang]

For debugging, it would be handy to preview the order confirmation page. Yireo seems to have a module that allows you to do this. I don't think we should roll this functionality into our codebase for fear of introducing a security hole but instead use their extension to preview it.
Our extension would check if the Yireo extension is installed and if so, would generate a preview link to their controller.

https://github.com/yireo/Yireo_CheckoutTester