R1:b35f538bc4cf

Use dateModified for Answer headers in Ponder

Summary: Fixes T9226, shows the update time, not the creation time.

Test Plan: Update an answer, see new date.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9226

Differential Revision: https://secure.phabricator.com/D13945