Twenty Fifteen, editor-style.css: revert the font-size back to 19px and margin back to 83px, merge the two body selectors (don't seem needed), remove redundant rtl rules and fix body.rtl selector. See #29799

git-svn-id: https://develop.svn.wordpress.org/trunk@29911 602fd350-edb4-49c9-b593-d223f7449a82