Fix card list group borders & radii

Created by: MartijnCuppens

Fix the top border in the card list groups (https://getbootstrap.com/docs/4.4/components/card/#kitchen-sink).

I've inherited the border-radius from the .list-group, this makes the selectors a lot less complex.

• https://deploy-preview-30497--twbs-bootstrap.netlify.com/docs/4.3/components/list-group/#flush
• https://deploy-preview-30497--twbs-bootstrap.netlify.com/docs/4.3/components/card/#kitchen-sink