R1:4135752490b8

Multiplex AJAX calls

Summary: Fixes T5344. Essentially, we only make the AJAX request to `/notification/individual/` if we are the leader tab (i.e. only one tab will make this request). Once a response has been received from the server (containing the contents of the notification), we broadcast the message contents back to all other tabs for rendering.

Test Plan:
Opened two tabs on `/notification/status/` and clicked "Send Test Notification".

**Before**
```lang=bash, name=tail -f /var/log/phabricator…