Separated rivet-source CSS from style.css

[zttodd]

In this PR:

• Moved the rivet-source Sass imports out of the style.scss file and into its own rivet.scss.
• Added Rivet core and component imports that are required for badges and modals to work.

Separating style.css into two files still adds up to roughly the same base file size before cache: ~190k. This update is mainly useful for inspecting CSS in the browser's developer tools, because the Docs-specific styles are in style.css and rivet-source CSS is in rivet.css.

The individual Rivet imports within the style.scss file seem a little untidy and is definitely duplicating CSS, so I'm open to discussion on this and/or whether this update is worth including some components twice. Could be something to work toward refactoring in future versions.

See issue #126 for more details.

[zttodd]

This will be considered when developing the next version of Rivet.