ECOM-811 design and layout changes

@wedaly @stephensanchez @rlucioni here are the style and text updates. Unfortunately I had to use teh dreaded !important in my Sass to override some ridiculously specific selectors, but after the A/B test we can clean this up by removing the legacy styles.
I'm surprised with this copy change -- I thought we were switching to using "enroll" for enrolling in a course and "register" for registering for EdX.