Update GSS codes for four councils
Update GSS codes for four councils

- East Hertfordshire, Northumberland, Stevenage and Gateshead.
- Our upgrade of MapIt meant that these GSS codes changed. This file is
  used to match SNAC codes, from MapIt, up with slugs so that frontend
  can take users to the correct result page for their council. It is
  also used to import local authority slugs into the new Local Links
  Manager, as of https://github.com/alphagov/local-links-manager/pull/4,
  and so the codes need to be kept up to date.

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend