Update footer links to current browse sections

Also update it so the list is in alphabetical order. As with the
homepage the easiest way of doing this is to have two lists floated side
by side.

I will update the other instances of the footer links once this has been merged/approved.

- https://github.com/alphagov/spotlight/blob/master/app/server/templates/page-components/footer_top.html
- https://github.com/alphagov/government-service-design-manual/blob/master/_includes/layouts/_footer_top.html
- https://github.com/alphagov/transformation-dashboard/blob/master/_includes/layouts/_footer_top.html