Move the review banner code out into a DraftReviewBannerView.

DraftReviewBannerView handles showing/hiding the banner in response to
events on the draft review, and handles the buttons on the banner.

DraftReviewBannerView is a singleton. It's created once up-front.
Callers can access the instance through
RB.DraftReviewBannerView.instance in order to show or hide the banner.

Manually tested all buttons, and various events that show the banner.

Unit tests pass.