Allow splitting off conversations / threads into own channel

[hloeung]

This is a wishlist item that I thought I'd run by first. It's spawned from the #matterircd channel on Libera.Chat.

Very often, there are certain threads where users are engaged in. It becomes a little complicated if it's in between or running concurrently with messages from various other threads.

Maybe we can add an option to have matterircd create private channels (local IRC channels with the & prefix per https://www.rfc-editor.org/rfc/rfc2811#section-2.2). Something like /msg mattermost breakout @@{THREADID} where matterircd would then create and a new channel, search for all messages in that thread and display it there (using the search function, or replay with a timestamp - #554).

Then any messages sent by the user in that channel are replies to that thread. Any new replies to that thread are displayed in this new channel (and perhaps in the main channel too).