Feature/series landings is fully operational

This significantly expands on the series "Landing Page" functionality, implementing meta-boxes on the Landing Page CPT editor per our wireframe for managing the contents and layout of the landing page.

In addition to a bunch of fairly straightforward meta fields that dictate layout options on series-landing.php, there's also some sophisticated functionality for managing the sort order. "Custom" is available once the Landing Page has been assigning to a particular Series term, at which point a lightbox should become available for drag-n-drop management of the order.

WordPress actually uses an ancient and crummy plugin (ThickBox) for the admin lightbox, so the lightbox itself doesn't look all that fabulous — but it should be good enough for what we need, I think.

Anyway, there's a lot here so please do play around a bit and ask questions (or identify bugs) as necessary.