Move to dart sass

[sbastn]

This PR removes Sass in favor of Dart Sass.

Closes https://github.com/dnsimple/dnsimple-developer/issues/175

QA

1. rake run
2. Open site
   • Layout, colors and font should match developer.dnsimple.com
   • Search a topic: matches developer.dnsimple.com
   • Browse an article: matches developer.dnsimple.com

[sbastn]

the preview looks like it failed to build, is that related to the gem change?

Yes. Something is wrong with the gem. The PR is not ready for review yet.