This is a squash of 38 commits ending with
5b080f979d692804452400ac5bed9b17c50b001e

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.

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

* reducing font-weight on component adding menu
* adding in and applying demi-strong font-weight
* adding in $color-copy-emphasized variable
* correcting stateful xblock settings styling nesting
* syncing video player xblock-based controls font-weight

STUD-2109

styling for recursive xblock rendering structure to be used in the container page, pulling from general xblock styles in a separate file.

Studio: adding back in links (and supporting styling) to ToS and Privacy Policy to footer and sign up UI
STUD-151

Also includes RequireJS changes.

Fixes bugs:
STUD-755, STUD-152, STUD-700, STUD-699

studio - fixes Sass variable defintion to ensure the older -serif references the proper new -sans-serif variable

typography, SASS import, mixins/extends

studio - sass cleanup: adding in more color variables, proofing box-sizing and older/rougher unit scss

studio - sass cleanup: local merge with master and small comment/organization tweaks to variables and base-styles

studio - tender widget: added compromise of style given iframe and clunky DOM elements that tender is using. meh.

studio - alerts: refactored styles for common content types across UI, got prompts set up and styled, and refactored base template for future prompt/notification/alert use

studio - alerts: converted transition-based animation to keyframe/css animation and added in a demo for a fleeting notification