feat(fonts): download fonts from grafana instead of CDN

Makes its possible to run grafana without internet access.
The performance hit by not using a cdn is very small since
most grafana instances are setup on locally.

closes #4223

public/sass/_grafana.scss

@@ -14,7 +14,6 @@
 @import "base/reboot";
 @import "base/type";
 @import "base/forms";
-@import "base/fonts";
 @import "base/grid";
 @import "base/font_awesome";