Copy translations when creating new draft of edition

Prior to this commit, creating a new draft of a published Worldwide Priority
available in multiple languages would lose the non-English translations.

I debated adding a cucumber scenario for this functionality but decided that
an additional test around `Edition#create_draft` was enough.
Move translation specific behaviour to a module

This follows the pattern we've established for containing related behaviour in
modules within the Edition namespace.
Show the language of each translation

Prior to this commit, we only displayed the language of the translation in the
'Edit the <language> translation' link, and that link isn't present for
published editions.

It makes sense to me that we should display the language of the translation
irrespective of whether the edition has been published.
Merge pull request #249 from alphagov/tidy-up-the-translated-edition-functionality

Tidy up the translated edition functionality

https://www.pivotaltracker.com/story/show/43569731