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 zh_TW
checking for parse errors