Support new data export infrastructure in Maniphest

Summary: Depends on D18958. Ref T13049. Support the new stuff. There are a couple more fields this needs to strictly improve on the old export, but I'll add them as extensions shortly.

Test Plan: Exported tasks to Excel, saw reasonble-looking data in the export.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18959