R1:fce43179e7b9

Move notifications to ApplicationSearch

Summary: Ref T5891. This just modernizes infrastructure.

Test Plan: Viewed "All" and "Unread" notifications.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5891

Differential Revision: https://secure.phabricator.com/D10281