Heartbeat API: add nopriv actions, add JS 'heartbeat-send' event, see #23216

git-svn-id: http://core.svn.wordpress.org/trunk@23481 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix typo in wp_heartbeat_settings, see #23216

git-svn-id: http://core.svn.wordpress.org/trunk@23482 1a063a9b-81f0-0310-95a4-ce76da25c4cd