LMS: edX-centric  New Footer Style/Markup clean-up

This work builds on the work done in https://github.com/edx/edx-platform/pull/4855 and is meant as a refactoring and clean-up effort. 

**It focuses on the following:**
* strongly revising markup used in edX-centric footer (moving away from Drupal-based output)
* strongly revising Sass used to style edX-centric footer (moving away from responsive, Drupal-based styling as well as removing !important and other heavy handed overrides)
* adjusting the naming conventions for template files

**Still to do:**
* Final visual proofing