Expand specialist document metadata from schema

When requesting extra fields such as case_type, we want to look up the label in the schema. This commit does that by passing around the index mappings, getting the extra fields and looking them up in the relevant schema. [Ticket](https://trello.com/c/UpIwl3zz/291-rummager-use-the-extra-schema-details-to-flesh-out-the-documents-response-3).
I think it would be good to check that the expanded field value here is correct.  eg:

    assert first_result["case_type"] == [{"label" => "Mergers", "value" => "mergers"}]