fix affiliated.astropy.org menubar

The affilated/index.html menubar for affiliated.astropy.org  was broken because it points to ``http://www.astropy.org/affiliated/*`` instead of ``http://www.astropy.org/*`` as it's supposed to.  This change fixes it by using sphinxs templating features to figure out the correct path.

Obviously I haven't tested it on the live web site, but it works for me locally.