Breakpoints and Ksl News: It's a Nav Group! It's a Dropdown! It's a Nav
Group!

+ Adds breakpoints in KslHeader component to set "size" state.
+ Adds modifier class during render.
+ Add prop to KslNews to specify how it should be rendered.
+ Pass prop to KslNews from KslHeader to render as NavGroup on large/xlarge
and Dropdown on medium.