Moved EditionFilter to own file
Extract partial
Move feature lists to a more general search UI

This isn't perfect yet: currently it will not check to see if a
translation exists for that particular edition before displaying it.
Ensure we always filter by translated editions

This ensures that when trying to search for something to feature, you
cannot choose something that hasn't been translated for the feature
list's locale.
Remove unneeded #featurable_editions method

Thanks @h-lame
Moved EditionFilter to Admin namespace
Merge pull request #495 from alphagov/featuring-articles-without-association

Feature any published edition on world location pages

Part of: https://www.pivotaltracker.com/story/show/47972795