SASS compiling in Python

[samuelcolvin]

The idea is to let developers customise the style of their app without needing to resort to npm/node.

We would need to:

• add libsass as a local dependency
• provide a utility for downloading scss libraries like bootstrap (I aleady have this logic in either harrier or grablib, can't remember)
• provide a utility for building the sass/scss
• provide an option to remove the default css import in prebuilt_html