Updates to make full-bleed secondary nav.

To be explicit and to decouple it from the base font size. The font size for `.nav-secondary_link` is 16px on small screens and 18px on larger screens. Let's say instead of saying 16px for small screens we just straight up use base font size. This will work fine for now. What happens if we ever change base font size? What will happen is that that on smaller screens the nav font size will change, but on larger screens it won't change. By using 16px and 18px respectively we can avoid a potential inconsistency.