Add skip links to both CMS and LMS

* add HTML for links
* hide links when they are not focused
* when focused, show link (as recommended)
* move accessibility tools to common

https://edx-wiki.atlassian.net/browse/LMS-1311

@adampalay 
@talbs (for the new HTML and SASS work)
@singingwolfboy (for the addition of a new JS library on the CMS side)