Advanced search for whitehall

Provide an advanced_search endpoint that can do querying as well as filtering and pagination.  The main use-case for this is to provide a better search experience for the whitehall search page which allows filtering the results by department, topic, world location, and document type, as well as reordering the results by date.

Takes: per_page, page, order, keywords params as instructions for pagination, query and sorting the result set.  All other params (ignoring app specific ones like format and backend) are assumed to be filter queries.  The request is rejected if these filter queries do not match properties in the mappings file for the index.

This pull request is supported by a corresponding pull-request to alphagov/gds-api-adapters#44 that provides the api client.