Save product prices and order totals

[proggeramlug]

Right now we are entirely relying on a product-manager instance to tell the order system the prices and to calculate the total.

While that is perfectly fine in many cases we still should save the price of a item at the time of purchase as well as the total at the price of ordering. If prices change we cannot track back what it was originally.

So we need to have two more fields: total for order and price for item (which is simply copied from product manager)