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. The work to create a builder which will consume
these pages can be found in slimmer.
Add caching to locale actions and basic tests
Make locale matching regexp match more locales

Locales can look like:

- en
- es-419
- zh-hk

Make the regexp match more things so all of these are valid.
Merge pull request #505 from alphagov/network-i18n

Add routes and files for network i18n requests