Use carmen to translate available_countries helper

[jhawthorn]

WIP: will revisit and test properly next week

In solidusio/solidus_i18n#106 we are looking to remove a feature from solidus_i18n which provides translations for each country based on the translations in the i18n_data gem.

We don't want to support this odd magic feature as part of i18n, and we'd like to remove the dependency on i18n_data, but at the same time we still want countries to be localized properly.

Ideally this would also be faster than the existing implementation as available_countries is a little bit slow.

This PR replaces the the existing behaviour of using I18n.t("spree.country_names.#{iso_code}") with using Carmen's data to perform translations.

This is the first time we are actually requiring and using carmen outside of tests and seeds, so we should consider the effect this has on startup time and memory usage.

TODO

• Continue to support overrides from I18n spree.country_names.*
• Change Carmen.i18n_backend with each request
• Benchmark
• Consider startup time and memory usage

[jhawthorn]

After

 available_countries    223.391  (± 4.5%) i/s -      1.116k in   5.006031s

Before

 available_countries     80.574  (± 6.2%) i/s -    406.000  in   5.057310s

> Benchmark.ms { require 'carmen' }
=> 6.585271989228204

[kennyadsl]

Thanks John!

[kennyadsl]

I'm preparing a PR on solidus_i18n to remove that feature.