Serving Fonts through Platform Assets

The fonts copied to ```cms/static/fonts``` and ```lms/static/fonts``` are:
[Open Sans "Webfont Kit"] (http://www.fontsquirrel.com/fonts/open-sans)
No subsetting.

Included fonts:
Open Sans Light, Open Sans Light Italic, Open Sans Regular, Open Sans Italic
Open Sans Semibold, Open Sans Semibold Italic, Open Sans Bold, Open Sans Bold Italic

Non-included fonts:  Open Sans Extrabold, Open Sans Extrabold Italic

Ticket for this PR is: [BLD-1228] (https://openedx.atlassian.net/browse/BLD-1228)

@talbs 
>Not sure if we need to give credit to font squirrel here - Open Sans is an open source font available across other sites as well.

Removed reference to Font Squirrel.