Replace RABL with presenters for search results.

Still needs a few tests on the search result presenter, and some
cleaning up on the URL helpers, but it works.
Add tests for the search result presenter.
Test the new URL helper class.
Remove the custom RABL error views.

There's so little information in them as they stand that it's easy to
reproduce them in straight Ruby.

As part of this, make the error response format more consistent: we were
previously using the "status" field both for short status names and for
longer-form error messages. Now we consistently have "status" for the
short status and "status_message" for longer errors.
Replace local authorities RABL with a presenter.
Reinstate description on local authority results.