Button Group fixes

[shawnbot]

This PR refactors our button group (BtnGroup class) component to:

• Use Primer SCSS variables instead of static px values
• Use display: inline-flex instead of display: inline-block + floats
• Add z-index: 1 to active (.selected, :focus, :active) button group items
• Replace hard-coded Storybook stories with those generated from the Markdown docs (previously, button groups weren't even present in Storybook!)

Fixes #600

[emilybrick]

the styling/css lgtm