Translatable publications

Publications are now translatable.  This includes new translation keys for the bits of the page we've made translatable (including some probably dubious French translations in order to allow tests to pass).

We also make it so that asking for a document that has been published in french (e.g. ``Document.published_as('slug', 'fr')``) will return nil, and this bubbles up to a 404 in the normal frontend controllers for each edition type.