Twenty Thirteen: clean up RTL and Genericons styles. Props SergeyBiryukov for the comment moderation bits, fixes #24287.

git-svn-id: http://core.svn.wordpress.org/trunk@24223 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: smarter margins for comment block-level elements that need to be centered. Use margin just for left and right; don't change top and bottom.

git-svn-id: http://core.svn.wordpress.org/trunk@24224 1a063a9b-81f0-0310-95a4-ce76da25c4cd