CSV Export / FIAT FX Rate

### Description
When exporting transactions into CSV file, the FIAT USD rate used is related to the date of export, instead of to the date of the actual transaction. The export FX rate is not useful at all, whereas the transaction date FX rate would be useful for accounting purpose.

### Version
0.0.3 Mac Version

### Steps to reproduce
Go into any wallet with transaction and export CSV

### Expected behaviour

Instead of date of export FIAT FX rate, use transaction date/time fiat FX rate.

### Actual behaviour

### Screenshots

### Device or machine
Mac

### Additional info