This is currently restricted by the ForumTreeConfiguration. If we remove the restriction, we need to make sure the forum and thread data (last/first message properties) is appropriately updated.

There are multiple cases to be (potentially) supported:

• move one message (and possibly its nested message) into another thread.
• create a new thread off of one message (and possibly its nested message) into a new thread. This can potentially be supported via 2 operations (1: create new thread 2: move messages) as long as the metadata is properly maintained.
• to avoid lots of clicking around, merging a complete thread into another might be good as well. (if the forum is configured to allow nested messages, this might be possible via moving the first message)