Enhancement: CSS classes for font weight

We added css classes to the design system which provide
font weights from 100 - 900:

.oc-font-thin: 100
.oc-font-extralight: 200
.oc-font-light: 300
.oc-font-normal: 400
.oc-font-medium: 500
.oc-font-semibold: 600
.oc-font-bold: 700
.oc-font-extrabold: 800
.oc-font-black: 900

https://github.com/owncloud/web/pull/8504
