Convert wallets into main currency

[junoslukan]

Would it be possible to select one main currency, into which all wallets in other currencies would be converted? In this way, the "total" wallet could actually show the total, sum of all expenses and incomes in one currency.

Since Open Exchange Rates API is already implemented, I guess making the call every time an expense is input should not be too much of a problem. As a first simplification, I think using their latest.json endpoint would suffice, especially since their historical/*.json is not free.

I am not sure, however, how much work this would mean for the backend, specifically the database and the models.