When viewing a Ponder question, clear notifications about it

Summary: Fixes T3703. Clear question notifications when viewing a question.

Test Plan: Gave a question a token, logged in as author, saw notification, viewed question page, notification was marked read.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T3703

Differential Revision: https://secure.phabricator.com/D6723