With the rewrite of the "Review" dialog, it was possible to create a draft

review by opening the dialog and closing it without making any changes. This

change adds some logic to avoid saving the review model if there haven't been

any modifications.

Without an existing draft review, opened the dialog and closed it. Saw no

  draft banner.
Without an existing draft review, opened the dialog, set "Ship it", and

  closed it. Saw a draft banner.
Without an existing draft review, opened the dialog, put some text in the

  body_top editor, and closed it. Saw a draft banner.
With an existing draft review, opened the dialog and closed it. Saw the draft

  banner stayed present.
Ran js-tests.