Generalise content storage and presentation for specialist docs

All document specific fields will be stored in a Hash field such that all
document types can be handled by a single mongoid class and presenter.

Adding more specialist document types should not require changes to the
Content API.

Depends on https://github.com/alphagov/govuk_content_models/pull/181

https://trello.com/c/bdpaMmOC/152-aaib-finder-publishing-aaib-reports-3