feat: highlight active footer navigation

[RNR1]

PR Checklist

• [x] Addresses an existing open issue: fixes #058
• [x] That issue was marked as status: accepting prs
• [x] Steps in CONTRIBUTING.md were taken

Overview

• Switched all footer navigation links + the back link from standard anchor tags into NavLink components.
• Added color: white; to active page-grid-footer-link navigation links.

[tjwds]

Wicked. Thanks!