Switch css build to sass within generated apps,

[duncdrum]

Using scss to extend the bootstrap would greately enhance the maintainability of generated apps, and allow us to transition update exist-2.2.css leftovers.

This requires a sass deps as parted of the generated packages, as well as porting the old css code into new scss modules extending the bootstrap provided base styles.

Lastly, this should come with additional tests, and the inclusions of styleint by default in the generated packages.