Allow archived attachments to be viewed

AttachmentVisibility was only allowing attachments for published
editions to be accessed for non-logged in users.

This commit adds a publicly_visible Edition scope which includes both
published and archived editions.
Merge pull request #1237 from alphagov/archived-attachments-fix

Allow archived attachments to be viewed

[Fixes https://www.pivotaltracker.com/story/show/64750912]