Bug 33863: (bug 25655 follow-up) Check "Change currency" only if checked before

Bug 33863: (bug 25655 follow-up) Check "Change currency" only if checked before

We should not check "Change currency" if it has not modified in the
original order.

Test plan:
Create an order with 3+ items
Receive 1 item, don't check "change currency"
Receive an other item
=> "Change currency" is not checked
Check "Change currency", modify the currency and the price
Receive a third item
=> Change currency is checked, invoice currency and price are set
correctly to the previous values

Signed-off-by: Michaela Sieber <michaela.sieber@kit.edu>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>