Check post locks with heartbeat and display modal notifications when a post is locked or a user takes over editing, props dh-shredder, see #23697

git-svn-id: http://core.svn.wordpress.org/trunk@23661 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Compress scripts/styles: 3.6-alpha-23662.

git-svn-id: http://core.svn.wordpress.org/trunk@23662 1a063a9b-81f0-0310-95a4-ce76da25c4cd