LEARNER-2977

Fixes

AC-727
began changing colors in course content, files and uploads, and when creating a course

finished fixing contrast issues on files and uploads page

fixed advanced settings and issue with hovering in files and uploads

fixed color issues in studio home page and when hovering

fixed contrast in updates, pages, textbooks, and group configurations

fixed issues when configuring a section of a course in course outline

fixed hovering colors to make contrast more visible

fixed colors on the green button

AC-727

* updating vendor files
* updating class syntax (to new FA-based classes) for all UI elements
* correcting broken tests

* removing unused font-family-centric Sass variables
* removing unused graphic images and Sass references
* clean up of style removal fallout

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

typography, SASS import, mixins/extends