Twenty Fifteen: Use a heading heirarchy to provide a better navigation experience when using screenreading software.

Props davidakennedy, rianrietveld, iamtakashi, afercia, davidakennedy, fixes #30065.


git-svn-id: https://develop.svn.wordpress.org/trunk@30072 602fd350-edb4-49c9-b593-d223f7449a82