New change message toast now contains the author name to avoid
reloading the change page in case the message posted is from bots
or irrelevant users.
However, the toast still persists permanently and remains a source
of distraction when users decide not to navigate away from the page.
Hence, add a button to dismiss the toast.
Change-Id: I7a4d19d267f5b87f1dd1822c41b302eb75693450