Add the description placeholder

Add the ``description`` placeholder (and the default behavior of
showing the page teaser) the ``cms_twocol.html`` template.

Addresses #426
Clean up section landing template

Don't repeat menu code, just inherit from ``cms_twocol.html``. Don't
show unpublished sub-pages.

Addresses #426
Implement menus for content re-org

Implement menus that show the current item and one level of children for
top-level pages and the ancestor and siblings for second-level pages.

Addresses #426, Closes #643