Post locks:
- Change from IDs to classes for the notification-dialog divs so they can be reused.
- Do not check post locks if the dialog's html is not present.
- Do not check post locks if there is only one user on a single site install.
See #23697.

git-svn-id: https://core.svn.wordpress.org/trunk@24304 1a063a9b-81f0-0310-95a4-ce76da25c4cd