Setup compass with bootstrap3 sass

This Pull Request creates a bigger change to the current frontend stack. To make it more transparent I created a WIP-PR :bowtie:. Tackling #2006 and later more of the styling issues.

Right now I changed the setup to use [Compass](https://github.com/Compass/compass) together with [Sass](https://github.com/sass/sass).

I hope you can provide me some early feedback so I know if this is what you expected.

- [x] Setup compass with bootstrap
- [x] Setup file structure for scss
- [x] Remove haml
- [x] Migrate all templates (layout x, ...)
- [x] Integrate other css to sass stack
- [x] QA to fix last glitches
- [x] Refactor scss (using shared variables etc.)
- [x] Adjust README.md (frontend setup)