Update organisation relationship text for exec agencies and NDPBs

Exec agencies and NDPB were being described as being 'of' their parent
organisation, eg. "X is a non-departmental public body of Y". Instead,
we should be saying that "X is a blah, sponsored by Y"
Orgs superseded by devolved administrations described as a 'body of Y'
Merge pull request #1671 from alphagov/modify-org-relationship-text

Modify organisationship relationship text