Move the test for #missing_translations

This tests behaviour provided by TranslatableModel so I'm moving to test
accordingly.
Introduce `TranslatableModel#non_english_translated_locales`

This should allow me to use this, and the related `#missing_translations`
methods in the person, role, world location and worldwide organisation
controllers, thereby DRYing up the index actions.
DRY up person_translations_controller

We can use the methods provided by TranslatableModel to DRY up this
controller.
DRY up role_translations_controller

We can use the methods provided by TranslatableModel to DRY up this
controller.
DRY up world_location_translations_controller

We can use the methods provided by TranslatableModel to DRY up this
controller.
DRY up worldwide_organisations_translations_controller

We can use the methods provided by TranslatableModel to DRY up this
controller.