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.
Merge pull request #292 from alphagov/metadata-schema-fields

Expand specialist document metadata from schema
Add document_type return field
Merge pull request #293 from alphagov/add-document-type

Add document_type return field
Add last_update field to ALLOWED_SORT_FIELDS

Allow it to be returned, and add the field to the schema too.
Merge pull request #294 from alphagov/allow-ordering-by-last-update

Add last_update field to ALLOWED_SORT_FIELDS