Conversion Codes input in the Shop Admin

I think we need an area in the Shop Admin to add in conversion codes for the checkout success page and maybe contact success page.

The feature will also need the ability to call information from the order.

For example, I'd suggest we use a shortcodes like so:
[ID] = Invoice ID number
[total] = The invoice total

It's only a 5 minute task for someone like me to add it in manually but it's something that gets requested so often that I think they should be able to do it themselves.