Unlike code comments, top-level comments on a change cannot be previewed
as a draft before they are published. This means reviewers do not get
the opportunity to ensure that their comment will be formatted as
expected. With this change, an optional preview feature is added to the
GR-REPLY-DIALOG.
Feature: Issue 5027
Change-Id: I186f973d5c178bb10128fac06c3dd51db5755c98