Use keywords instead of ints to update task priority in ManiphestEditEngine

Summary: Fixes T12124. Changes ManiphestEditEngine to populate the select using priority keywords instead of the integer value. Marks maniphest.querystatuses as frozen. Adds a new Conduit method for fetching potential task statuses.

Test Plan: Created tasks and changed their priorities, observed that transactions in the DB still have the same type (integers as strings). Invoked maniphest.update with priority => '90' and observed that it still works. Invoked maniphest.edit with priority => 'unbreak' and observed that it now works.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T12124

Differential Revision: https://secure.phabricator.com/D18111