Add support for the pre-payment webhook

[adamjudd]

Now that Foxy has the pre-payment webhook, support for this should be added to FoxyShop too.

By default, it should support checking inventory levels, and preventing a checkout if a customer is trying to checkout with a larger quantity than currently exists.

Example stand-alone implementation can be seen here: https://pastebin.com/raw/iCAaddg5, although this doesn't differentiate between limited stock and out of stock.