Only show translated editions on world location pages.

I've generalised and extracted the `translated_into` factory helper
and included it into the `world_location`, since we don't really
care about what the translated attributes are, just that a valid
translation exists.
Replace explicit translation creation with factory usage.

I believe extracting the way translations are created will make the
tests less prone to churn as the translation implementation evolves.

This commit uses the new `translated_into` trait in another test, and
enhances that trait to allow setting of custom translated values where
important for a test.
Merge pull request #226 from alphagov/only-show-translated-editions-on-world-location-pages

Only show translated editions on world location pages

Part of https://www.pivotaltracker.com/story/show/42513563