Export to publishing api

With the move of GOV.UK to use the Content Store, this PR contains commits which allow us to now publish Finders to the PublishingAPI. It contains a `PublishingAPIPublisher` class which handles exporting the Finder and it's signup page, a `ContentItemPresenter` class which has 2 child classes for the Finder and the Signup page. It also contains a spec for the PublishingAPIPublisher and a rake task to trigger publishing.

This changes to the schema add `content_id` which is needed to publish to the content_store. This has been generated in the console as with such a small number of Finders we don't need to worry about generating them in the App. I've also added the organisations and related (if required) fields to tag the Finders with their Org and other, related content_items.

This PR also bumps gds-api-adapters to get the Publishing API removes `better_errors` as this is an API only App an we don't need it.

Ah, sorry, just realise that these are JSON files so making them constants isn't possible...