* add HTML for links
* hide links when they are not focused
* when focused, show link (as recommended)
* move accessibility tools to common

LMS-1311

- 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

Verification: converts all class-based Sass archetype-focused @extends to use Sass placeholder syntax

The official border-radius mixins were deprecated and removed in Bourbon 3.0.
https://github.com/thoughtbot/bourbon/pull/95

lms - adds back in missing/previously added .sr screen reader-focused styling and references in place of the .visuallyhidden application

edx.org - resolved (in a dirty way) iframed buttons picking up style in IE and added in coming soon style treatment

The help tab opens a modal dialog that directs the user at various resources
(e.g. the site FAQ and course forums) and allows the user to submit feedback
to the feedback endpoint (which will ultimately create a ticket for the
student support team).

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

lms - revised classes and styling mechanics for login/register forms and added necessary screen-reader mixin for other visually hidden text in those pages

drupal integration - revised general page background for shorter content pages, small visual tweaks to footer

drupal integration - revised padding between content and footer globally, removed secondary form actions for time being

Rearranged more images, made some smaller, made some slight changes to the design on the homepage and made all edX proper case

renames some scss files for better orginization, sets fixed widths on main containers to avoid akward scaling issues untill media queries can be added for more responsive scaling