Feature/landings

Implements "hierarchical" display of posts on category/term/taxonomy pages (that aren't otherwise overriden by the "series landing" functionality. 

To do this, we implement two new terms in the "Post Prominence" taxonomy, "Featured in Taxonomy" (replacing the placeholder "Featured in Category" term) and "Secondary Featured in Taxonomy". Those terms are used to place the appropriate posts in the top, middle or normal list part of an archive page. While a single post could appear in both the top and middle if both of these terms are checked, no posts in either of these featured terms will appear in the traditional listing underneath.

We also simplified the archive.php template a bit to "make room" for all this new display logic. It may be a good idea to build a separate author.php file to manage author archive pages, rather than having any author-specific logic mixed in with this hierarchy stuff.