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.
Long labels like "Unstar messages" can be confusing for
translators and it can also create bad strings that can
end with like 4 long words in German. It is better to
have the simple options like "Confirm" and "Cancel".
This commit fixes this issue by changing such text
to "Confirm" as default in confirm_dialog_modal.
Part of #17926.