Twenty Thirteen: fix text overflow with sidebar in IE8, props obenland. Fixes #23579.

git-svn-id: https://core.svn.wordpress.org/trunk@23610 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: move IE-specific properties out of main stylesheet. Props obenland, see #23507.

git-svn-id: https://core.svn.wordpress.org/trunk@23611 1a063a9b-81f0-0310-95a4-ce76da25c4cd