Expand the current site template with two columns in the header

There are a lot of designs that have a right column in the header where they usually add things like a search box and a language switcher. We should expand the site template so that it has a right and a left column in the header that can be used to for instance have the logo in the right column and the search box in the left column without writing CSS that is specific to these panes.

We should keep the current region (branding) above the two columns so that any existing layout will continue to work unaffected.