Handle multiple totals in different currencies (OCC-64)

[DemeSzabolcs]

Part of #16 followup to #127. We can only pay in one currency at a time. Later we have to figure out what to do if there are multiple totals i.e., multiple currencies. A solution could be to convert the totals in other currencies to one.

Jira issue

[Piedone]

Do we really need this? While having the ability to use anyone currency in a shop is necessary, and later perhaps we can add the ability to add prices in multiple currencies and be able to switch between them (or rather, convert automatically) as just a nice to have feature, I don't see when doing a checkout with multiple currencies in the basket would make sense.

[sarahelsaig]

Depends on what you mean by "this". :) The feature not urgent at all, but we need the issue "for the record". It's a known undefined case that could potentially cause problems and an issue is better than just a comment in the codebase.

[Piedone]

"This" = having multiple totals :). The checkout should calculate (i.e present to the user) prices in a single currency. If we have a situation where there are items with multiple currencies in the DB then we should handle that somehow properly indeed.