Notifications - add a "Clear All Notifications" link to dropdown

Summary: Fixes T2576. Also hyperlinks "Notifications" and "Messages" for easier quick navigation to those areas. Maybe we could get rid of the "See All X" UI at the bottom and use these links?

Test Plan: cleared all notifications from new UI - it worked! observed new linked "Notifications" and "Messages" headers

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T2576

Differential Revision: https://secure.phabricator.com/D8894