Round out the button family with the two navigational siblings described
in the UI guide: Anchor renders a styled <a> for external links and Link
renders a react-router link for in-app navigation, both sharing Button's
tv styles. Add an `active` look variant so nav items can carry a
persistent selected state without forking structure.
Signed-off-by: Émile Ré <emile@probo.com>
a6be5464a5