LMS/Accounts: Converts many class-based Sass archetype-focused @extends to use Sass placeholder syntax

edx.org: revises course register button background for all cases (verified, riught column, and bottom of page)

edx.org: adds background to register button's html body element for visual blending in course detail view

If a user is already registered for a course but the course has not started
yet, display a grey button that says "you are registered" and goes nowhere
instead of the blue "access courseware" button, which goes to the LMS course
about page (which 404s).

edx.org - reduces padding on course listing register buttons (all states) and resets box-sizing to border box to make a proper box model for element

edx.org - resolved (in a dirty way) iframed buttons picking up style in IE and added in coming soon style treatment