DRY up use of scopes to find latest editions

There are plenty of controllers that want to find the latest few
editions of a specific type (e.g. the 3 latest policies that have been
published in a world location). There's plenty of duplication in those
chained scopes, so we can tidy things up a fair bit by moving them into
a method and setting appropriate defaults.
Allow #latest_presenters to filter by current locale
Use ternary operator instead of if/else
Don't link to english docs from wales-office.cy

When viewing the Wales Office organisation page in Welsh, you really
don't want to be reading links to documents in English. We filter all
publications and announcements, showing the latest ones that have been
translated, rather than just showing the latest ones in Welsh OR English.
Drop pointless method wrapping a scope

Somebody might quote Demeter at me, but I'll risk it.
Merge pull request #761 from alphagov/wales-office-links-to-welsh-docs

Wales office links to welsh docs
https://www.pivotaltracker.com/story/show/54278086