DO NOT MERGE 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](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.
Could the `k` here be changed to `facet_key` to be more descriptive?