Move the reply draft banner out into its own view.

The reply draft banner has been moved out of reviews.js and into
ReviewReplyDraftBannerView. The new code is more "dumb" in that it
doesn't deal with anything beyond rendering and handling the
publish/discard. It's up to the caller to determine what happens after
(such as page reloading).

The floating support moved into FloatingBannerView, which
ReviewReplyDraftBannerView inherits from. This has been made a bit more
generic, so that it can be use for other banners later on.

Tested all the floating semantics. We're bug-for-bug compatible with
what we had before all the JavaScript refactoring (tested using RBCommons).

Tested that the banner appeared after a new comment.

Tested both publishing and discarding, both after an initial new comment (when
there wasn't previously a banner), and after a page load with an existing
draft comment.