Minimal required translation to view an International Priority.
Add a test to verify .with_translations behaviour works.

It's worth noting that .with_translations will only return editions where *every*
attribute has been translated; they query explicitly discards translations that
have NULL-value fields.
Only show priorities with translations on world location pages.

As part of this, I've added the minimum set of spanish translations
to render an international priority using my test data, but this may
still not be enough to avoid falling back to English in production.

I've also added a general mechanism to the Edition factory for
creating translated versions:

    create(:published_policy, translated_into: [:fr])
Merge pull request #225 from alphagov/only-show-translated-priorities-on-world-location-pages

Only show translated priorities on world location pages

Progress towards: https://www.pivotaltracker.com/story/show/42513563