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

I think we should be whitelisting the valid locales, just to be extra safe, especially as it ends up loading a file from the file system and passing it through `YAML::load_file`.