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 don't think this should be here. We should handle this in the same way as we do with `ALLOWED_FILTER_FIELDS` being combined with schema fields