As of #23867 this is now a real package, so updating the imports to
use the real package name, rather than relying on the alias. The
name change in the package name is because `I18n` is not a valid
name as NPM packages must be all lowercase.
This commit also introduces an eslint rule to prevent importing from
the old I18n path.
For themes/plugins, the old 'i18n' name remains functional.
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
