Fix notifications happening for your user in a different tab

If you open an instance of the Faction web app in one tab, then open it in another and send a message while logged in as the same user, it will send a desktop notification.

Reproduce the two-tab scenario described in the issue, then trace the web app's desktop-notification handling and the message events shared between tabs. Done means sending a message as the same user in one tab no longer triggers a desktop notification in the other tab.

Written by the indexing model from the issue text.