The dialog now shows `stream > topic` instead of
just the topic.
This creates a new tiny widget for this purpose,
which can be reused in other places too.
The `undefined` case is probably very unlikely, but
if the `stream_id` is bad, after the user confirms,
the request will fail on the backend, which could
confuse the user, since there will be no changes to
the starred messages in view.
So, we don't open the confirmation dialog at all in
such cases.
Abhijeet Prasad Bodas