This is a squash of 38 commits ending with
5b080f979d692804452400ac5bed9b17c50b001e

ECOM-2114

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.

About pages need basic support for lists since professors often want to display lists or 'definitions'

- 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

With tests, some settings changes
(all should default to not breaking anything for edx)

Added styling for shopping cart User Experience
- Styled shoppingcart list page
- Styled navigation shopping cart button
- Styled receipt page
- Styled course about page for shopping cart courses

Addressed HTML/SCSS issues

Remove offending body class and unnecessary sass changes

Addresses many review comments on stanford shopping cart

* framework for generating order instructions on receipt page
  in shoppingcart.models
* move user_cart_has_item into shoppingcart.models
* move min_course_price_for_currency into course_modes.models
* remove auto activation on purchase
* 2-space indents in templates
* etc

revert indentation on navigation.html for ease of review

pep8 pylint

move logging/error handling from shoppingcart view to model

Addressing @dave changes

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

Now that we are using separate pages for login and registration rather than
modals, clicking the registration button for a course should direct an
unauthenticated user to the registration page, and the user should be enrolled
in the course upon successful registration. Likewise, an unauthenticated user
attempting to unenroll from a course should be directed to the login page and
subsequently unenrolled from the course upon successful login. The enrollment
change service also now uses HTTP status codes rather than JSON to communicate
status to the front end.

lms - cleared document flow for .teacher elements within a course's about view to coincide with Studio's default HTML for the course overview settings field