R1:8008ade9af46

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…