1. "What Goes Up Must Come Down" – if you subscribe to message bus, make sure you also unsubscribe
2. When you unsubscribe - remove only your subscription, not **all** subscriptions on given channel
Attempt #2. The first attempt tried to extend a core `@bound` method in new-user-narrative plugin which did not work. I reworked that plugin in the meantime. This new PR also cleans up message bus subscriptions in now core-merged chat plugin.
1. "What Goes Up Must Come Down" – if you subscribe to message bus, make sure you also unsubscribe
2. When you unsubscribe - remove only your subscription, not **all** subscriptions on given channel
This will allow consumers to inject it using `siteSettings: service()` in preparation for the removal of implicit injections in Ember 4.0. `site-settings:main` is still available and will print a deprecation notice.
