Twenty Fourteen: implement tag auto-complete for featured content settings, and uppercase `__CLASS__` for consistency. Props kovshenin, see #25549.

git-svn-id: https://develop.svn.wordpress.org/trunk@25813 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fourteen: cleanup, props iamtakashi. Fixes #25613.

 * With the ephemera widget in place, `content-recent-formatted-post.php` is unused, see #25028.
* We're no longe filtering formatted posts from the main query, see #25330.
 * Fix a typo.

git-svn-id: https://develop.svn.wordpress.org/trunk@25814 602fd350-edb4-49c9-b593-d223f7449a82