Twenty Thirteen: second pass at styles for attachment pages that are not images: hide sidebar and navigation link, better alignment of Edit link.

Closes #23632.
Twenty Thirteen: fix missing space in selector! Props markmcwilliams, fixes #23652.
Twenty Thirteen: trim down font properties, props obenland. Fixes #23546.
Twenty Thirteen: reduce font-size a bit for comment content areas. See #23651.