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
Use published_at as the "Updated at" to display.

This means we control when the document is shown as having been updated,
not mongoid.
All AAIB metadata is singular

aircraft_types and registrations are actually singular everywhere except
here. Hence they weren't displaying as they were nil.
Merge pull request #41 from alphagov/fix-pluralisation-of-aaib-metadata

All AAIB metadata is singular
Fix links to docs from pre filtered Finder

When linking back to a pre-filtered Finder, the URL has a trailing slash
which then breaks links to documents as they are set as
`finder-path/doc-name` which it assumes are then at
`finder-path/finder-path/doc-name`. This commit removes the trailing
slash.
Merge pull request #42 from alphagov/fix-links-to-documents-from-pre-filtered-finder

Fix links to docs from pre filtered Finder
Make dt metadata tag wider

With the introduction of the DFID finder we have some longer <dt> tag
that go make the <dd> tags not in line. This commit makes the <dt> a
bit wider to account for this.
Merge pull request #43 from alphagov/make-dt-metadat-tag-wider

Make dt metadata tag wider
Merge pull request #40 from alphagov/use-published-at-not-updated-at

Use published_at as the "Updated at" to display.