Nav System

[LHinson]

Introduce the PF4 nav system: https://pf-next.com/components/NavSystem/examples/

Interaction specs can be viewed here:
https://github.com/patternfly/patternfly-next/wiki/Horizontal-Navigation
https://github.com/patternfly/patternfly-next/wiki/Vertical-Navigation

In addition to a React review, this issue should be reviewed by the core contributors of this demo:

• CSS: @mattnolting
• Design: @kybaker @mcarrano