Twenty Fourteen: redo the featured image background pattern with CSS plus minor style adjustments.  Props iamtakashi, fixes #25300. This also fixes #25363, props buffler.

git-svn-id: https://develop.svn.wordpress.org/trunk@25610 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fourteen: fix gap beneath header image, props taupecat. Fixes #25404.

git-svn-id: https://develop.svn.wordpress.org/trunk@25611 602fd350-edb4-49c9-b593-d223f7449a82