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

ECOM-1547 created new template and Sass file for new footer. Wrapped it in a feature flag and updated previously named footer-edx-new.html to footer-edx-v2.html to better track/name updates to the footer going forward.
ECOM-1547 removed data-updated attributes from body
ECOM-1548 added JS file that inits analytics event listener and makes ajax call to API to get footer and HTML and add it to the DOM
ECOM-1547 code review cleanup
ECOM-1547 rebase and Sass appraoch update after @talbs the IE9 killer slayed the mighty beast.
ECOM-1547 updated file names and added comments in light of @talbs review of PR
ECOM-1547 update to social media link styles and nav link order
ECOM-1547 updated SOCIAL_MEDIA_FOOTER_NAMES if ENABLE_FOOTER_V3 is true
ECOM-1547 added translations to screenreader text
ECOM-1547 fixed test and renamed files in line with @talbs's pending PR
ECOM-1547 cleaned up file naming