LMS: Use Open Sans Typeface via Google Web Fonts

This work syncs up how the LMS loads Open Sans (a typeface we use throughout edX's UI) to how its loaded/referenced elsewhere (Studio, Certificates, code.edx.org) - through Google Web Fonts. Key changes include:

* uses Google Web Fonts (http://www.google.com/fonts#UsePlace:use/Collection:Open+Sans) reference rather than local files
* removes unused local font files and license (originally downloaded early 2012 from fontsquirrel)
* adds in semi-bold weight (600) support