fix header link font sizes
fix header link font sizes

/cc @emplums: I started with a verbatim copy of the header from
primer/components, but after introducing the global primer CSS it looks
like I've had to set fontSize={2} explicitly on all of our links to bump
it back up. The way I did this feels kinda yucky; would you have done it
differently?

primer/primer,primer/primer,primer/primer-css,primer/primer-css,primer/primer,primer/primer-css