Allow Finders to be published to the PublishingAPI

With the move of GOV.UK to use the Content Store, this commit allows 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 class and a rake task to trigger publishing.
Add content_store fields to Finder metadata

As Finders are now published to the content_store they need some extra
metadata fields. This commit adds the content_id for each Finder to be
sent along with arrays of content_ids of Organisations and Related
content items.