Section landing page template

This commit adds the ``cms_section_landing.html`` template, a template for
Django CMS pages that live at the top of their section.  By default, it displays
the teaser of the page and lists all the child pages along with their teasers.
These regions can be overridden by specifying plugins for the placeholders.

In order to access the teaser in the template, I had to patch the ``Page``
 model with a ``get_teaser`` method and patch the ``PageAttribute``
class (which implements the ``page_attribute`` template tag so it considers
``teaser`` and allowed attribute.

Finally, I made a custom manager for ``Teaser`` objects to implement the same
language fallback pattern as the native ``Title`` objects.

Addresses #426
List child pages in two columns.

Addresses #426
Merge branch 'develop' into 426-child_promoting_theme
Merge remote-tracking branch 'origin/develop' into 426-child_promoting_theme
Refine section landing page template

Renamed the ``teaser`` placeholder to be ``description`` to
better reflect @jwirfs-brock's
[comment](https://github.com/PitonFoundation/atlas/issues/426#issuecomment-14180196)
about how the content on these pages is structured.

I also adjusted the grid system classes to make the list of sub-pages
align correctly.

Addresses #426
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show unpublished sub-pages.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