Add locale column to attachments
Rename edition to attachable in attachments partial
Allow editors to set locale on attachments

Also added Locale.coerce to simplify dealing with symbols, strings and
value objects and rewrote Locale#initialize to coerce input (e.g. strings)
to symbols.
Only show attachments for the current locale

If an attachment has no locale set it is shown for all request locales.
Merge pull request #1116 from alphagov/attachment-locale-for-translations

https://www.pivotaltracker.com/story/show/52384573

Attachment locale for translations