Matrix: make forum answers to fold into a thread

[mordamax]

From original request:

Forum notifications in one channel and threading for posts
implemented here: https://github.com/discourse/discourse-chat-integration/blob/a73f5da114d6d7adfac53bdd58b7f45a4e3ac31a/lib/discourse_chat_integration/provider/matrix/matrix_provider.rb
Answer to forum posts from Matrix

Problem:
Currently forum posts every individual post or answer under it as separate message in matrix room, which is (i suppose, from request) hard to read.

Solution:
The idea is to leverage standard (yet beta) threads functionality of matrix. So when someone posts a new topic, it goes directly to matrix room, and when someone answers under topic - it automatically places answers in thread under topic message

Additionally someone suggested that would be nice to be able to answer to the forum topic from the matrix thread. So if you post a message in this thread from element client - it'd automatically reply from your account in forum. But this needs to be evaluated separately. (Do we really need it, what's the effort and caveats)