Add mw-ui-checkbox

Move checkbox styling from BetaFeatures extension to core.
Drop vendor prefixed border radius support (http://caniuse.com/border-radius)

Use adjacent selector which is supported IE 7 and greater
Use not selector to filter out IE < 8 which do not support :checked

Change-Id: I6a0db7c8ce33d242120f1cba9222db4e2154696c
(cherry picked from commit 0ea24459f3e0b83b02cfabd40da2205e4601b9f9)