fix(buttons): disable danger styling if disabled

[austingreendev]

Description

@vedranio found an issue when users choose to add disabled styling a <Button while also using the danger styling.

We should add some disabled logic checks to our conditional className logic.

Fixed Version

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 💂‍♂️ includes new unit and snapshot tests
• 📒 any new files are included in the packages src/index.js export
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[vedranio]

👍 for the design review