Twenty Thirteen: adjust line-height to be consistently applied across elements, props chriswallace and obenland. Fixes #23514.

git-svn-id: http://core.svn.wordpress.org/trunk@23497 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: style comments correctly when `show_avatars` option is turned off in Discussion settings. Props SriniG, fixes #23558.

git-svn-id: http://core.svn.wordpress.org/trunk@23498 1a063a9b-81f0-0310-95a4-ce76da25c4cd