Add maniphest.priority.search method
Summary: Start on plan outlined in T12124. Adds a new Conduit method for querying information about task priorities.
Test Plan: Ran locally; observed expected output:
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D18035