- to house anything we need across multiple apis

- addresses_with_authority_data returns an array of the addresses with
  local authority slug and name, suitable for presenting from the app.

- this is genuinely an API, so rather than serving it as a separate format on the
  existing /find-local-council endpoint, we just make it its own thing, which
  allows us to restrict it / update it independently of the other controller. Since
  we've simplified the code in the existing find_local_council controller,
  the index method in the new controller is basically a direct copy of the
  find method - you're expected to call it with a query parameter, there isn't
  currently a simple index that lists all local authorities.
- the query is a GET - in the web interface it's a POST, but that's largely
  for privacy. The lookup is extremely cacheable at the CDN level - it might
  change less than one time a year - so we might as well take advantage
  of that.
- status codes are 301 (redirect straight to a local authority url to be added
  in a subsequent commit), 200 (a list of addresses to choose from if the postcode
  crosses boundaries), 404 (postcode is structurally valid but not found in the
  database), or 400 (postcode query parameter is missing or not valid).

… the parent if present.

- This is much simpler, at this point we're just doing slug lookups and returning
  a 404 or the local authority hash (including the parent hash if there is one).

…w apps)