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: {F4979109}

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D18035