Allow extra metadata for AAIB reports.

In allowing both filterable metadata (aka metadata present in the
schema) and extra metadata (data not present in the schema and
translated at the presenter layer), we (me and @evilstreak) decided to
return an object representing each item of metadata rather than a hash.
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.