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

Work by @tommyp (6d4e656):

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](https://github.com/alphagov/gds-api-adapters/pull/185) 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.

Relies on https://github.com/alphagov/finder-frontend/pull/47 being merged to parse the radio facets correctly.

Work by @kalleth (80a6766):

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 decided to return an object representing each item of metadata rather than a hash.

Also relies on the pull requests mentioned above.

This supercedes pull request #37 

To be completed:

- [x] Split 80a6766 into two commits
- [x] Merge and release https://github.com/alphagov/gds-api-adapters/pull/185
- [x] Update Gemfile to point to the new gds-api-adapters version
I think `filterable` would be better than `filtered` here, for consistency with the `filterable_metadata` method.