Store invoice paid currency on successful checkout
Currently, when invoice is paid, there is no way to check in what currency customer has paid.
A new `paid_currency` field should be added, which will be filled in in `tasks.py` on successful checkout.
This also requires an edit to [invoice page headers](https://github.com/bitcartcc/bitcart-admin/blob/master/pages/invoices.vue#L19) in admin panel to add a new field