Twenty Fourteen: match `screen-reader-text` accessibility styles a bit better...

Twenty Fourteen: match `screen-reader-text` accessibility styles a bit better with core, to avoid a jump when you move through the links, and to avoid a horizontal scroll at certain viewport widths. Also fix JS focus by specifying the first match object. Closes #25592.
Built from https://develop.svn.wordpress.org/trunk@26843


git-svn-id: https://core.svn.wordpress.org/trunk@26730 1a063a9b-81f0-0310-95a4-ce76da25c4cd