Customize: Add changeset locking in Customizer to prevent users from overriding each other's changes.

* Customization locking is checked when changesets are saved and when heartbeat ticks.
* Lock is lifted immediately upon a user closing the Customizer.
* Heartbeat is introduced into Customizer.
* Changes made to user after it was locked by another user are stored as an autosave revision for restoration.
* Lock notification displays link to preview the other user's changes on the frontend.
* A user loading a locked Customizer changeset will be presented with an option to take over.
* Autosave revisions attached to a published changeset are converted into auto-drafts so that they will be presented to users for restoration.
* Focus constraining is improved in overlay notifications.
* Escape key is stopped from propagating in overlay notifications, and it dismisses dismissible overlay notifications.
* Introduces `changesetLocked` state which is used to disable the Save button and suppress the AYS dialog when leaving the Customizer.
* Fixes bug where users could be presented with each other's autosave revisions.

Props sayedwp, westonruter, melchoyce.
See #31436, #31897, #39896.
Fixes #42024.


git-svn-id: https://develop.svn.wordpress.org/trunk@41839 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/customize.php

@@ -87,6 +87,7 @@ add_action( 'customize_controls_print_styles',         'print_admin_styles', 20
  */
 do_action( 'customize_controls_init' );
 
+wp_enqueue_script( 'heartbeat' );
 wp_enqueue_script( 'customize-controls' );
 wp_enqueue_style( 'customize-controls' );