[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
- [x] ~~use the same library for [deployment](http://dahlia.kr/libsass-python/frameworks/flask.html#building-sass-scss-for-each-deployment)~~ (in production we set `GITTIP_CSS_HREF` to point to MaxCDN which caches the request for us)

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