Add extra preview-mode selector to the CMS actions so we can show
something when the preview is closed (and with it all options are not
visible).
Thanks @mateusz, @clarkepaul and @robert-h-curry for contributing.
Introduces a preview window that appears when the screen is wide enough,
removes old preview button, adds a draft-public switch, adds variety of
preview options which are not hooked up yet.
Goes along with cms commit fa3738a9f4c5181eabf18a77ca89792d31592250
