Use the correct layout of posts for LMP posts

[benlk]

Changes

• creates a new partial partials/content-region.php for use in Load More Posts queries for the region taxonomy, which implements the "row of three span4, row of two span6" layout using an ugly global hack that is the only way to implement it without rewriting Largo's LMP :frowning_face:
  • this requires, in the RFC 2119 sense of the word, that the number of posts to display on archive pages be a multiple of 5.
• filters the LMP partial-selector to use that partial for LMP region archive queries
• copies Largo's mechanism for having the first post on a category archive page be outside the main Loop, and be a featured item from that term or just the most-recent item from that term

Why

• so that LMP-loaded posts match the layout of posts from initial page load
• so that the top post on the term archive page isn't duplicated

[benlk]

This PR is complemented by creating the post prominence "Featured in Region" with the slug 'region-featured', to match what the code expects for the top post query.