Support discussions

As my observation, people want to talk or discuss topic in notes.
And that might cause the note like a mess.
There was an idea to make or integrate some chat system.

But now I think Disqus is another great choice.
To make the experiment better, will also make it SSO.
https://help.disqus.com/customer/portal/articles/236206-integrating-single-sign-on

The issue names no files, tests, or entry points; start by reviewing the linked Disqus SSO guidance and the existing discussion thread. Define the discussion and SSO scope, then verify that the agreed discussion experience works in notes.

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