Upgrade Sass

[ericwbailey]

Our current version of Sass is outdated, and its syntax will cause breaking changes when updated. We'll need to:

1. Update Sass to the latest version.
2. Verify that Netlify's new VM can process it.
3. Update all old Sass syntax to use new Sass conventions.

This issue comes from a conversation with @mxmason. We'll also want to reference @esjay's work on #1141, as this may affect that.

[ericwbailey]

Closed via #1390.