`discourse-common` was created in the past to share logic between the
'wizard' app and the main 'discourse' app. Since then, the wizard has
been consolidated into the main app, so the separation of
`discourse-common` is no longer useful.
This commit moves `discourse-common/(lib|utils)/*` into
`discourse/lib/*`, adds shims for the imports, and updates existing
uses in core.
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
