Add new pages. Change top nav.

index.html: Revise top level nav bar. Edit HTML to make divs stretch
fully across page and be fluid.

glossary.html: New page. Add collapsible accordion functionality for
glossary terms.

uncertainty.html: New page. Add draft text about data uncertainty.

download.html: New page. Create shell for inserting download procedures.

about.html: New page. This page links from “Historical Sea Ice Atlas”
in top nav.