Add localised routing to people

This ensures that /people/<person-slug>.<locale> works correctly. Previously,
you would've had to add the '?locale=<locale>' querystring to view a person in
a different locale. This also ensures that links to people take you to the
page in the locale you're viewing the current page in.

*NOTE* I've had to extend the `stub_translatable_record` helper so that the
`#translations` method responds with something that can answer to `#where`,
which in turn responds with something that can answer to `#exists?`. This is
because methods (specifically, `#available_in_locale?`) now comes into play
when generating the route for a person.
Exclude 'biography' from `Person.columns`

As in d1fc876aae4e7a8dbb8ca74e4b80452102e1e4e4, this will enable us to
gracefully remove the biography column in a future commit (the biography now
comes from the person_translations table).
Exclude 'name' and 'responsibilities' from Role.columns

As in d1fc876aae4e7a8dbb8ca74e4b80452102e1e4e4, this will enable us to
gracefully remove the name and responsibilities columns in a future commit
(these values now comes from the role_translations table).
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.