Fulfilled order price might change in time

[dtwm]

• longclaw version: 1

In current implementation order price is determent from current product price. https://github.com/JamesRamm/longclaw/blob/a041d3d505539f03dce4d0570f067ecc1eb4e7a9/longclaw/orders/models.py#L91-L93

Product price might change with time, value of paid (or accepted) orders shouldn't. This also will bite once sales and discounts will be introduced.
btw, relation to product via on_change=DO_NOTHING might lead to unexpected results.