Remove defunct translation keys, etc.

* Remove everything from `config/locales/en.yml` that couldn't be found with ack commands in `missing_translations.rb`

* Add translation for country_names.US

* Additional translation tweaks
Make translations in backend controllers, helpers, and views now use Spree.t
Remove frontend routes from backend
Require correct file in i18n_spec
Fix translations across api, core and frontend
Add missing translations for mail_method_settings, quantity, stock_locations and stock_movements
Namespace dash translations
Re-add shipping translation
Translate descriptions for shipping calculators using Spree.t
Move activerecord translations out of spree namespace
Add hook for after request specs to show which have missing translations
Add more missing translations
Add missing partial admin/orders/_shipment