Updating the platform header styles to match new site styles.

Putting the new edx header behind a feature flag.
Merge pull request #4879 from edx/sanchez/update-platform-header

Updating the platform header styles to match new site styles.
Replace reference to Google OpenSans with edX-hosted OpenSans
Addressed comments.
Fixed path to font files.
Use Bourbon mixin utility instead.
Ordered font declarations by increasing font-weight