* This feature is currently only visible to admins.
* Locally echoed messages are also updated.
* Add UI for editing stream if user is admin.
* Show propagate mode selector if either stream or topic changed.
This completes the implementation of support for moving a topic to
another stream by adding a basic UI for it.
Fixes#6427, which was previously the most-upvoted issue request in
Zulip.
There are likely to be a bunch of follow-up UI improvements on top of
this change to fully flesh out the feature.