Moves the search box into the header.

I think for this to work, we would need to make a search modal rather than reusing the sidebar to show search results. The issue with reusing the sidebar is that on mobile, the sidebar isn’t visible when searching (and would obscure the search box if we open it).

We’d also want a way to show the Observable logo in the top-left where currently it says Observable Framework. And perhaps on mobile, we’d want to show this logo in the header, and collapse the search box to an icon.