Multicurrency transaction support

Add an ability to note the original currency of the transaction. It shouldn’t affect any calculations, it’s rather for the tracking itself, and for the future reporting tools.

The case:

1. Account is in USD

2. Base currency is in EUR

3. User travels to Japan and spends in JPY

Right now: JPY expense on the USD account is deducted in USD, so no JPY information stored anywhere

Expected new behavior: User has an ability to manually add information about the original currency of the transaction

Request details: https://github.com/letehaha/budget-tracker/issues/416