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

Props philiparthurmoore, fixes #30296



git-svn-id: https://develop.svn.wordpress.org/trunk@30302 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: create consistent spacing with embeds and images.

Props afercia, fixes #30286



git-svn-id: https://develop.svn.wordpress.org/trunk@30303 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fifteen: simplify the code used to generate screen reader text for the is_home title.

Props davidakennedy, fixes #30305



git-svn-id: https://develop.svn.wordpress.org/trunk@30304 602fd350-edb4-49c9-b593-d223f7449a82