Studio: refactoring font-size to use Sass mixin
* moving font-size to use @include font-size() mixin
* re-organizing vendor reset styling into _vendor.scss override rather than _reset.scss
* initial rule re-organization and minor line-height tweaks
Studio: refactoring font-size to use existing Sass scale-based placeholders
Merge pull request #4791 from edx/talbs/studio-stylesweep-fontsize

Studio: Refactoring font-size CSS rules to use Sass mixins/placeholders