Twenty Fifteen: simpler CSS by using divs instead of spans for menu descriptions.

Props philiparthurmoore, fixes #30296


Built from https://develop.svn.wordpress.org/trunk@30302


git-svn-id: http://core.svn.wordpress.org/trunk@30301 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: create consistent spacing with embeds and images.

Props afercia, fixes #30286


Built from https://develop.svn.wordpress.org/trunk@30303


git-svn-id: http://core.svn.wordpress.org/trunk@30302 1a063a9b-81f0-0310-95a4-ce76da25c4cd