Support custom fields in "Order By" for Maniphest

Summary:
Resolves T4659.  This implements support for sorting tasks by custom fields.

Some of this feels hacky in the way it's hooked up to the Maniphest search engine and task query.

Test Plan: Queryed on a custom date field, with a small page size, and moved back and forth through the result set.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4659

Differential Revision: https://secure.phabricator.com/D10106