Add routes and files for network i18n requests

Add routes and files to enable network based i18n requests for strings.
This will mainly be used by components but is generic so it could be
used for anything. 

These pages will be needed for this change in slimmer: https://github.com/alphagov/slimmer/pull/101

Some locales have more than two characters, e.g. `es-419`. See https://github.com/alphagov/content-store/blob/master/config/application.rb#L37-L42 for the locales we want to support.