Deduplicate application-level notifications from Aphlict

Summary:
Fixes T12564. We already had some code which seems to deal with this properly, it just wasn't getting used.

Assign each application-level notification a unique ID, then ignore messages with duplicate IDs.

Test Plan:

• In browser A, loaded /T123.
• In browser B, loaded /T123.
• Made a comment as B.
• Saw notification as A.
• Mashed "Replay" a bunch.
• Before patch: piles of duplicate notifications.
• After patch: no duplicates.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12564

Differential Revision: https://secure.phabricator.com/D17710