Mark answers as page objects in Ponder

Summary:
Fixes T4306. We should clear notifications about a question and its answers when viewing a question page.

(Eventually we might have an answer detail page and send the notification there, and then only clear there, but this cleans things up for now.)

Test Plan: Loaded question page, verified answers appeared as page objects.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T4306

Differential Revision: https://secure.phabricator.com/D7928