Make Notification menu abs positioned

Summary: Fixes T5537. Makes it so I can scroll and see the bottom of notifications.

Test Plan: Open Notification Menu. Scroll. No longer follows me.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5537

Differential Revision: https://secure.phabricator.com/D10261