Add locale awareness

This adds a new field to a content item to signify the language of the content. It also includes a data migration to update all existing content with the default, which is "en" for English.

This is part of some wider work to allow translated content to be served and linked to in Publishing 2.0.

Story: https://trello.com/c/lbF0TQmy/50-push-all-translations-of-whitehall-case-studies-to-the-content-store