hgweb: use separate CSS class for navigation links in footer 2d93d2159e30 changed the styling of the "page_nav" CSS class to use flexbox to separate elements within the <div>. I didn't realize that this class was used outside of the links in the header. So this resulted in incorrectly formatting links in the footer of various pages. Fix that by introducing a new CSS class that preserves the old CSS behavior.

#require docutils gettext

  $ $TESTDIR/check-gendoc fr
  checking for parse errors