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:**

* naming conventions for template files
* strongly revising markup used in edX-centric footer
* strongly revising Sass used to style edX-centric footer (removing !important and other heavy handed overrides)

**Still to do:**

* Proof this and the previous work in IE9
* Optimize files/assets (logos)
* Determine the appropriate place for separate edX-specific HTML (currently separate) and edX-specific styling (through theming)