Custom megamenu markup

@jwirfs-brok didn't like the inconsistency of the large teaser as shown
with the ``legend`` class.  So, the teaser text gets replicated here and
split into a heading and paragraph.

An improvement would be to find a clever way to split up the first and
subsequent lines of the teaser.
Render top and footer navigation from the menu

Instead of hard coding link text and URLs in the upper-right-hand corner
and footer, menus, generate them using the ``show_menu`` template tag.

I like this because it breaks out chunks of markup in ``base.html`` a
bit more and should get rid of annoying execptions when a page doesn't
exist in the CMS.

It also makes the footer menu automatically match the megamenu.