Use the newer hash response format for searches

This is part of upgrading all clients, and then later we can make the hash
response_style the default.
Gracefully handle the q param being empty

Until recently we could rely on gds-api-adapters, which would return an empty
array if the query was blank. Now though, we are expecting a Hash, so we get an
errror.

With this change we check the param ourself and reply accordingly.
Remove unnecessary search params

These things are now the default, since:
https://github.com/alphagov/rummager/commit/1b0ab48073f758eb914c82471edee95c7e0477f2
Add prettier tests with the `turn` gem.

Also reports test timings, so we can pick out slow-running tests.

Conflicts:

	Gemfile.lock
Remove licence edition

This is not used by odi and is cluttering up codebase. Get rid of it.
Remove business support editions

Not used and not relevant to odi. Deleting.
Remove local authority and transactions

Not used and not relevant to odi. Deleting it.
Remove travel advice.

Travel advice and foreign travel advice is not relevant for odi. Deleting it.
Missed some local authority bits.

Deleting.
Remove places.

ODI doesn't use or have imminence so no need for places. Deleting it.
Remove unused countries.yml
Add first-pass result set presenter class.
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.
Reinstate description within results presenter.
Create single result presenter
Replace tag-listing functionality.

This results in a fair bit of duplicationg between the new URLHelper object
and the old URLHelpers module; we can remove that once we've removed the _tag
template from RABL.

Cherry pick conflicts:

	govuk_content_api.rb

We don't want local transactions presenter. We don't need it.
Display null parent field if tag has no parent.

This was the behaviour of the API before de-RABLing; if we do change it, we
should do it deliberately.