Add the ability for course staff to specify a Survey and make it required so that a student must fill it out before starting the course

Co-Authored-By: Brian Talbot <btalbot@edx.org>

TNL-549

UX-351

* provides new .scss file for developers to add temporary/in-progress Sass for further refactor/polish
* cleans up note/import order in all application scss files

- Doubled up :hover and :focus styling to improve a11y
- Increase contrast of certain UI elements for improved a11y
- Added some image alt text for a11y
- Changed video caption styling to blue and made them underline on hover and added a skip link before video for screenreaders.

Fixes Bugs:
- LMS-1336

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