Legalpad - add support for querying by contributors

Summary: Fixes T3479

Test Plan: queried for contributors and got good results. tried a complex query with all possible values specified and got results when appropos.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3479

Differential Revision: https://secure.phabricator.com/D6384