Update overflowBoundaryElement usage to use React's createPortal

[interactivellama]

Now that React 16 features can be used. Components should update to createPortal.

https://reactjs.org/docs/portals.html

This will simplify the Dialog code and remove the need for context explicit passing to Portal children.

This will also allow Enzyme testing out of the box.

[mtetlow]

This will also resolve issues with multiple nested overflowBoundaryElement components using the new PortalSettings component ( https://github.com/salesforce/design-system-react/pull/2867 )