Feature 119 scss infrastructure++

*updated version of #387*

As a first step to refactor HTML and CSS (#119) this PR creates the infrastructure to port the existing CSS to SCSS.

Next steps will be:

* split CSS to multiple folders and files
* refactor CSS using advanced SCSS and CSS3 features
* refactor HTML

This will also change how theming is done.

I already worked on some of that in [feature-119-scss+](https://github.com/xi/adhocracy/tree/feature-119-scss+), but those commits still need a lot of clean up before they can be merged.