Optimize available_countries

Use .sort_by so that we only run .parameterize once per country.

Fixes #4899
Added missing translation for back_to_shipping_categories_list

Fixes #4993
Do not blindly accept just any month for CreditCard expiry

Fixes #4971

Conflicts:
	core/app/models/spree/credit_card.rb

Conflicts:
	core/config/locales/en.yml
	core/spec/models/spree/credit_card_spec.rb