- 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

Bourbon's box-shadow mixin has been deprecated, and they recommend to use the
official spec.

The official border-radius mixins were deprecated and removed in Bourbon 3.0.
https://github.com/thoughtbot/bourbon/pull/95

media kit - added in new tab/window behavior for download links, revised edX name styling on homepage cta, and provided more context on logo download text

[# 41872203]