Add footer, improve MyUSA button, clean up CSS/HTML

For Javascript clients,
- When the content doesn't fill the whole page, the footer will be pinned to the bottom of the page
- When the content fills the page, the footer will scroll with the content.

For non-Javascript clients, the footer will flow on the bottom of the content, c'est la vie.

Make header fullscreen like the footer (so the line goes across the page)

Improve the MyUSA button and the nav pils.

Add tests from #166.