website: Fix URLs.
    
    Enable local export to link URLs correctly.
    
    * website/www/utils.scm: New file.
      (current-url-root, gnu.org-root): New variables.
      (base-url, gnu-url, guix-url, static-base-url, css-url, image-url)
      (thumb-url, screenshot-url, slides-url): New procedures.
    * website/www.scm (main-page): Use them.
    * website/www/about.scm (about-page): Likewise.
    * website/www/contribute.scm (contribute-page): Likewise.
    * website/www/donate.scm (donate-page): Likewise.
    * website/www/download.scm (download-page): Likewise.
    * website/www/help.scm (help-page): Likewise.
    * website/www/shared.scm (html-page-header, html-page-description)
      (html-page-links, html-page-footer): Likewise.