Don't create a draft review if nothing is changed in the "Review" dialog.

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.

Testing done:
- 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.

Fixes bug 3590.

Reviewed at https://reviews.reviewboard.org/r/6537/