Make viewing a user's profile page clear notifications about that user

Summary: Ref T13222. See PHI996. This is a general correctness improvement, but also allows you to clear test notifications by clicking on them (since their default destination is the recipient's profile page).

Test Plan: Clicked a test notification, got taken to my profile page, saw notification marked as read.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19867