Redirect attachments on unpublished editions

Instead of showing the placeholder page for attachments on unpublished
editions, we redirect to the unpublished edition's old slug, which
will render the unpublishing information to the user.
Merge pull request #719 from alphagov/unpublished-attachments

Redirect attachments on unpublished editions

https://www.pivotaltracker.com/story/show/52377493