Admin should not restrict countries to checkout_zone

[jhawthorn]

From @softr8 in #solidus slack

I need to create some stock locations with a different country than the one available for the storefront(Spree::Config.checkout_zone), is this some special case for me or this is something that would be more general?

This seems like a bug. In both frontend and backend we use available_countries to build a list of countries.

There's a Spree::Config.checkout_zone setting, which is intended to limit the countries which can be chosen on the frontend. Because this helper is shared and also used on the admin, it also limits the countries which can be seen and selected there.

We should change this so that the admin can always select all countries.