Serve branded footer JSON/HTML/CSS/JS from an API endpoint
in the branding app.  Refactor OpenEdX and EdX.org footer templates
to use the Python version of the API, ensuring that the API
values are consistent with the footer included in main.html.

Detailed changes:

* Added footer API end-point to the branding app.
* Footer API allows the language to be set with querystring parameters.
* Footer API allows showing/hiding of the OpenEdX logo using querystring parameters.
* Deprecate ENABLE_FOOTER_V3 in favor of the branding API configuration flag.
* Move no referrer script into main.html from the edx footer template.
* Rename rwd_header_footer.js to rwd_header.js
* Cache API responses.

Authors:
Awais Qureshi, Aamir Khan, Will Daly

* simplifying edx.org breakpoints to use min-width
* creating -bp-* centric Sass variables
* converting all existing edx.org centric media queries to use new Sass variables

ECOM-1457

(SOL-531)(SOL-532)Course Cards update; A link to course discovery page has been added to the OpenEdX homepage

Fix link stylings in LMS and CMS footers

Allow configuration to override links to mobile app stores

Remove Jobs, News
Change "Vision" to "About"

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.

 Made the header and footer responsive

Currently the .container and .wrapper-footer elements are set to 100%,
and also have horizontal padding added to them. This results in a total
width of greater than the full width of the viewport; This causing the
browser to scroll horizonally into empty space, regardless of how wide
the viewport is.

I had added the `box-sizing: border-box` to these elements in order to
resolve this.

Cheers,
Louis

Move the open edx logo inside the div

Adding a bunch of placeholder views.

indenting.

Making some styles work against LMS sass.

Adding back the old edx footer and associated icons, with a feature flag.

Reuse the style from login and register views.
Remove all the specific style.

We need to use an Open edX footer by default (and that's what all non-edx.org
instances should base their footer off) and only use the edX footer on
edx.org.

This commit uses `SITE_NAME` to determine which footer to use.

Sites using theming or micro-sites with a `SITE_NAME` not ending in `edx.org`
should not be affected.

- 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

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

lms/mktg - removes margins from social icons and reinstates flex-gutter margin to colophon area of footer

edx.org redesign - revised temporary login/register views to fit with design requirements/fixes from marketing team's overall design changes to site's visual theme