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

## Tasks before merging
- [x] Deploy the changes to Specialist Publisher in https://github.com/alphagov/specialist-publisher/pull/141
- [x] Republish all CMA cases so they include the new details hash