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.
`_.sprintf` should be used to interpolate the platform name here, instead of hardcoding "edX." You'll have to pass in the platform name to this step in `pay_and_verify.js`.