Allow handling of AAIB Reports

Allow finder-frontend to display different types of reports, including old legacy documents with document_types of 'specialist-publisher'. 

This also introduces some custom mapping of date metadata attributes of a document to short length labels (not pulled from the schema!) and allows date labels to be visible on the frontend rather than hidden from view.

This will introduce some changes to '/cma-cases' too although these are merely for consistency with new design views. For example: dates are now displayed with labels on both '/aaib-reports' and '/cma-cases' 

Depends on: https://github.com/alphagov/finder-api/pull/28