Configure CNC on transition app

Do not merge until this organisation has been created in whitehall. https://www.gov.uk/government/organisations/civil-nuclear-constabulary
`civil-nuclear-police-authority` is a parent of [`civil-nuclear-constabulary`](https://www.gov.uk/api/organisations/civil-nuclear-constabulary), so you don't need to use `extra_organisation_slugs` here because the parent org's members will be able to edit mappings for the site anyway.