Generalise publishing for all document types

* Core attributes remain first class (slug, title, summary, body)
* All other attributes are added to 'details' field
* Other attributes are selected from the finder schema
Legacy publishing adapter to ease transition

The new generalised publishing set up puts all document attributes beyond title,
body, summary, slug in a details hash field.

This adapter flattens and duplicates the data export such that it will work
across the transition period.
Merge pull request #141 from alphagov/generalise-document-publishing

Generalise publishing for all document types