Accessibility on MdModal

[kajsaeggum]

Describe your changes

Added functionality to MdModal to fulfill the accessibility requirements of a dialog:

• role "dialog"
• aria-modal "true"
• aria-label describing the dialog contents
• allow closing of the modal using Escape-key

Also added forwardRef to MdLink, to be able to focus on a link-component. Specifically to be able to bring focus back to the correct place after closing a MdModal.

Checklist before requesting a review

• I have performed a self-review and test of my code
• If new component: Is story for component created in stories-folder?
• If new component: Is tsx-file import added to packages/react/index.tsx?
• If new component: Is css-file added to packages/css/index.css?