Replace Link from react-router with Link prop in Header and Footer component

[jaronheard]

What

For the following components designed to be used outside of civicplatform.org

Footer
Header
NavRouterLink (used in Header)
NavSubMenu (used in Header)

Instead of directly using Link from react-router, require a Link component to be passed in as a prop.

For components that use Header and Footer that are used solely within civicplatform.org (e.g. PageLayout), directly use Link from react-router here instead

Why

So that Header and Footer can be used outside of civicplatform.org
hackoregon/education-splash-page#9

When

ASAP, before Demo Day