Add some DfT sub-organisations that can edit the DfT mappings

- This adds extra_organisation_slugs to DfT so that those orgs can
  start using the transition tool from day one when is deployed. This
  list has been confirmed with DfT to be the only orgs they want
  editing their site's mappings (i.e. these orgs have sites inside
  dft.gov.uk).
Add driver-and-vehicle-standards-agency as a new extra organisation
Merge pull request #611 from alphagov/dft_extra_organisations

Add some DfT sub-organisations that can edit the DfT mappings