ProductVariant multiple prices for different currencies

[michaelbromley]

Currently a ProductVariant has a price property with a single integer value.

It may be desirable to allow a ProductVariant to have multiple price values, one for each supported currency.

• One way to achieve this would be to use Channels (see Channels #12) with a separate Channel representing each currency zone.
• Another approach is like that of Moltin products where the price field is an array of currency-value objects.

In practice, both of the above imply a one-to-many relationship of ProductVariant -> Price objects, in which case we may as well opt for a Channel-based approach, since Channels also unlock other desirable features.

[michaelbromley]

This is now made possible by the underlying implementation of Channels (#12) but still needs the admin ui part.