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