Add dates to notifications page

Summary: Fixes T3957, adds timestamps to the notifications page.

Test Plan: View my notifications page, see the new time stamps. Uncertain if I set $user correctly.

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: Korvin, epriestley, aran

Maniphest Tasks: T3957

Differential Revision: https://secure.phabricator.com/D8039