Move font license warning into mixin

[mikewheaton]

The purpose of References.scss is to provide a reference to all of the variables and mixins, without generating any output in the built CSS file. We're doing a good job of this with one exception: the warning about using the Fabric assets, such as icons and fonts. This comment is included in any built CSS file that imports our references.

Steps to fix:

• Move the comment into the ms-font-face() mixin, so that it is only included in the CSS when that mixin is used. See this CodePen demo of how CSS comments can be included in SCSS mixins.
• Identify other places that the CDN is referenced and make sure those have appropriate warnings as well. The warning should be included whenever we have a reference to the CDN, and at no other time.