Studio: correcting font weight/display issues

* reducing font-weight on component adding menu
* adding in and applying demi-strong font-weight
* adding in $color-copy-emphasized variable
* correcting stateful xblock settings styling nesting
* syncing video player xblock-based controls font-weight

STUD-2109
Merge pull request #4904 from edx/talbs/studio-fix-font-weight

Studio: FIX - Font Weights
Replace reference to Google OpenSans with edX-hosted OpenSans
Addressed comments.
Fixed path to font files.
Use Bourbon mixin utility instead.