Link back to pre filtered finder

When viewing a document, it's useful for the metadata values to link
back to the Finder with that filter pre selected.

This commit relies on the corresponding change in gds-api-adapters which
alters the metadata hash to have a slug as the key and inside that
contain the label and an array of values, each of those containing a
label and a slug.
Refactor to allow extra (non-schema) metadata.

In doing so, we also return an OpenStruct rather than a hash to ease
behaviour below the presenters (views and helpers).
Add extra metadata for AAIB reports
Link back to Finder from format name
Bump gds-api-adapters to 13.0.0

This change includes the ability for the Finder API to translate
attributes to user friendly values while keeping their slug for linking
back to the Finder.
Display large metadata values wrapped in <dd>

Large metadata values shouldn't be joined using to_sentence. This commit
rectifies that, removes the unused _metadata partial and sets the markup
in the header for the metadata values as it was.
Merge pull request #39 from alphagov/link-back-to-pre-filtered-finder-rebased

Link back to pre filtered finder + add extra metadata for AAIB reports