* Adds a `notifications` property to instances of `wp.customize.Panel` and `wp.customize.Section`.
* Adds a `setupNotifications()` method to `Panel`, `Section`, and `Control`.
* Adds a `getNotificationsContainerElement()` method to the `Panel` and `Section` classes, like `Control` has.
* Replace hard-coded notification in header media section with a notification.
* Limit rendering notifications to panels and sections that are expanded, and to controls that have an expanded section.
See #34893, #35210, #38778.
Fixes#38794.
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