Discussions: close a discussion

Motivation

Usually, a discussion is being created to discuss something with a goal. Once this goal has been achieved, the discussion is going to be deserted. Semantically, it's "done".

Proposal

The same semantic already exists for a room: "Archived". Once a room is archived, it can still be read, but subscriptions are hidden (so that the side panel is getting cleaned up) and the room is write-protected.
The "archiving" function could be re-used, it just needs to be made more prominent. A dedicated button either next to the message area or in the room properties (maybe even replacing leave/hide) would help.
In addition to that, the status "closed" should be visualized on the discussion-started-message in the parent room.