FIX: prevents registering multiple `topic-notifications-button:changed` (PR #9356)

A large topic page will always have the bottom tracking button, and will also have the timeline, meaning we already had 2 tracking events.

But it gets even worse when you know that the timeline button is a component connector which will trigger didInsertElement very frequently, meaning we were constantly adding more and more appEvents handlers.

GitHub

This pull request has been mentioned on Discourse Meta. There might be relevant details there:

https://meta.discourse.org/t/tracking-a-topic-using-the-keyboard-shortcut-tracks-other-topics-too/143790/14