Modals should be inside Portals

To prevent Modals being accidentally cut off by parent styles we should wrap the modal component in a Portal, like we do for contextual menus:

https://github.com/canonical-web-and-design/react-components/blob/master/src/components/ContextualMenu/ContextualMenu.tsx#L196

We'd need to figure out how to control the display of the Modal as the functions to display/hide the modal would be inside the component:

`const { openPortal, closePortal, isOpen, Portal, ref } = usePortal({`