R1:eaecf35324b3

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…