Conversion of threads into discussions

With reply-based threads almost in the wild (#13996), I can already see the following need:

Motivation

Once multiple threads which tend to become long-runners within the same channel have been created, it is becoming more and more difficult to navigate them.
Also, I'd like to be able to join an expert into a particular thread without having to add them to the entire private parent channel, which contains sensitive information

Proposal

Allow conversation of a thread into a discussion. Once this is executed (todo: by whom should this be possible? Proposal: Owner, moderator of the parent room as well as the author of the initial message), the messages should be made available in a separate room. All participants who replied inthe thread shall be the members.
This operation does not have to be reversible, imho - though it was nice if the messages of the discussion could still be visualized inline for convenience.

Start by reading the reply-based thread work in #13996, then trace how threads, rooms, and participants are represented. Resolve who may convert a thread, whether conversion is reversible, and how inline messages should appear before implementation; done means the behavior is specified and covered by tests.