Allow separate threads with same members

Igor Mashevskyi → made their first commit to this issue’s fork.

artem_sylchuk → changed the visibility of the branch 2942602-unable-to-create to hidden.

I've asked ChatGPT what is the common behavior of the most popular instant messaging apps

See https://chatgpt.com/share/678a43be-dc5c-800a-beea-6453c73b29db

I don't remember the exact history of this issue, but as long as it was bumbed, here is what I remember:
1. It is impossible to force the uniqueness of a thread for a given set of members. For example, if user 1 and user 2 have a thread and there is another thread between user 1, user 2, and user 3, by simply removing user 3 from the site, we'll create a collision. There are other possible cases.
2. It may make sense to have two separate things: private conversations between two users and chats with multiple members. Those can be bundles of private message threads, with different "unique" policies.
3. There is another request and "feature" that we probably may want. The thread uniquness may depend on something else except the thread members. For example, it can be limited by some node that references the thread. The idea was to have getThreadEntityForMembers event that can be used for overriding the thread, but it lacks the context.