(SOL-776)(SOL-740)(SOL-532) Added setting flags for COURSE DISCOVERY feature; Exposed course search input as well as the course result page; Added flag for limiting the number of course cards on the homepage;

SASS compilation variable conventions update, focusing on basic colors, baseline variable usage for padding and margins, ui-depth extends, along with other small fixes. The intent with this is to not have any user-facing impact, and instead move us closer toward having files that use our newer sass conventions.

LMS-2121

- 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 background-size mixin has been deprecated, and they recommend to use the
official spec.

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]

[# 41872203]

Rearranged more images, made some smaller, made some slight changes to the design on the homepage and made all edX proper case