Feature request: Save reference to exchange rate and original amount in foreign currency on bill

[mbbert]

Please save the original amount in foreign currency and a reference to the exchange rate on the bill.

The 2 new fields should appear when the project has at least one exchange rate configured.

The converted amount in home currency should be editable as well and should not affect the original amount in foreign currency nor the exchange rate when changed. This way it is possible to enter the final amounts which where charged e.g. on the credit card billing, incl. commissions or surcharges, etc..

When changing a rate in the currency list, it should be possible to recalculate the home currency on all bills where the exchange rate was used, with the new exchange rate. This could either be implemented with a new dedicated button or a dialog "Update all bills with this rate? Yes/No" when saving the new rate.