TinyMCE: Updating Google Font Syntax

With our upgrade to Sass a bit ago, we can now use plain CSS imports (Google's preferred syntax for embedding their web fonts) This work, building on the solid work of #1975, updates our font file import syntax in common/static/css/tiny-mce.css to :

* match syntax used when importing Open Sans in LMS/Studio
* simplify and move syntax to modern Google Web Font standards
* maintain the http/https agnostic work