Autosave to the browser's sessionStorage, compare this autosave to the post content on page load and let the user restore it when the data is not the same. First run, see #23220

git-svn-id: http://core.svn.wordpress.org/trunk@23683 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Compress scripts/styles: 3.6-alpha-23684.

git-svn-id: http://core.svn.wordpress.org/trunk@23684 1a063a9b-81f0-0310-95a4-ce76da25c4cd