The CSS that handled the icon-based actions on the left-hand side of the header

(star and archive/mute) was pretty ugly. We had a mix of padding/margin

specifications, especially working around a bad value in icons.less for the

position of the archive icon. We also had rules spread around in various

places, including a couple that were completely obsolete.
This change includes the first introduction of review-request.less. My eventual

goal is to move everything corresponding to the review request box into there,

leaving reviews.less for the actual review/change description boxes.

Verified the actions' appearance in Chrome, Firefox, and Safari.