refactor(button-group): add radio layout

[dancormier]

Note This PR branches off of the work done in https://github.com/StackExchange/Stacks/pull/1038

This PR adds an option to use radio elements for .s-btn-group. In the process of building this, I cleaned up the CSS/Less and removed the .s-btn-group--container element altogether since it a) convoluted the CSS and b) wasn't being used in Core anywhere.

More TBD.

[netlify[bot]]

✅ Deploy Preview for stacks ready!

Name	Link
🔨 Latest commit	bbe84a3bedd8a36c8cb334f31f6a0f417dc3781e
🔍 Latest deploy log	https://app.netlify.com/sites/stacks/deploys/636575ef865d5e0009e25629
😎 Deploy Preview	https://deploy-preview-1135--stacks.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[dancormier]

Gonna merge this into dcormier/btn-css-refactor despite a minor hover background-color bug. It's a bug that can only be resolved in btn styling, so I'm leaving it for the btn work.