Card feature updates

[castastrophe]

Incorporate a few new features into the card layout:

• Support for background color in the header region
• Update variable names and add additional hooks
• Add overflow setting to allow children in slots to fill the card
• Add storybook support for self-closing tags (img, br, hr, etc.)
• Updates border-radius to 3px globally

[starryeyez024]

@castastrophe after running npm run clean && npm install, the command npm run storybook returns an error at localhost:9001 that says:
Story of "Call to action" named "pfe-cta" already exists

[starryeyez024]

http://pastebin.test.redhat.com/718499

[castastrophe]

These updates look good 👍

[kylebuch8]

Alright. I think we're finally good to go with card. Just a heads up, we're making a few changes to the PFElement base class.

In the connectedCallback(), we're adding this.setAttribute("pfelement", "") and then in the _mapSchemaToSlots method, we're using this.setAttribute(has_${slot}, "") if the slot exists.

This is all done because set classes might be fragile.

View this comment to see what helped guide us to these changes.

[kylebuch8]

Licking Gorilla Tastes Mango