[WIP] Setup compass with bootstrap 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
- [ ] Migrate all templates
- [ ] Customize bootstrap template
- [ ] Integrate other css to sass stack
- [ ] Setup file structure for js/coffeescript and use bootstrap there
- [ ] Adjust README.md (frontend setup)