SCSS/CSS refactor

Created by: koenpunt

I could sum up al my commit messages here, but basically it comes down to a more uniform SCSS and some more declarative naming of mixins and variables. And my main reason to do this; the clutter of different definitions of classnames on elements. Before half of all were separated with underscores _ and the other half with dashes -. Now its all dashes.

Ultimately there should be some kind of code quality control in GitLab for SASS like https://github.com/twitter/recess for LESS