QuietButton follow-ups (private component)

[Enrico Sacchetti]

Caused by #361

In an effort to get @tds/community-quiet-button into a stable state, also known as v1, let's address the following:

React component changes needed:

• change px units to rem units
• Recommend Box instead of hard-coded padding
• use core-colours instead of hard-coded colours
• Consider not using cursor: pointer, though that requires a holistic effort across all buttons
• Can leverage core-button prop types for A11yContent