Add head and footer to the 404 page

[sharrocksb]

Add header and footer to the 404 page. The footer doesn't need to be visible in the viewport, but it should be there to give navigation options.

Per David: possibly needs the addition of an empty

and

html in your 404.html but that should be it?

[sharrocksb]

@pfulton - did you add the header and footer to the other pages? And do you know if it's an easy process?

[cjgammon]

@davidnuescheler is there a way to use blocks or existing components in the 404 page? I'm curious because we have a lot of styles and components built for the header elsewhere and it would be ideal to reuse those here in the 404 page.