[1758] Use Python-based SASS library for compilation

Resolves #1758
- [x] move over to a Python SASS library so we don't have to rely on the Ruby version, and hence can include it as part of the devlopment environment
- [ ] use the same library for [deployment](http://dahlia.kr/libsass-python/frameworks/flask.html#building-sass-scss-for-each-deployment)
- [ ] make it possible to load jQuery from the local server (and make it the default)