Deduplicate application-level notifications from Aphlict
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.
- 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.
Reviewed By: chad
Maniphest Tasks: T12564
Differential Revision: https://secure.phabricator.com/D17710