Show translation preview links in single button

When an edition is translated into multiple languages make the user
choose a translation from the main preview button. This replaces the
individual preview buttons which were in the translations section of the
admin edition show page.
Nilify blank attachment locales before saving

If a locale is saved as an empty string it doesn't match
`where(locale: [nil, :fr])`, used in the Attachment scope
for_current_locale.
Merge pull request #1134 from alphagov/preview-translated-editions

Preview translated editions via the main preview button