Twenty Thirteen: better font-size rules all blockquotes, setting a smaller base rule and overriding the large only for entry content. Also fix too-wide comment iframes.

Props Viper007Bond for the nudge. Fixes #23651.

git-svn-id: http://core.svn.wordpress.org/trunk@23548 1a063a9b-81f0-0310-95a4-ce76da25c4cd