This is a follow up on r41732, implementing the following improvements:
* Attachment parent info is now stored in attachment meta rather than a
separate post meta key.
* Attachments created from contextual crops (e.g. header, logos, etc.) are
filtered out of the media library using a new `_filterContext` method in
`wp.media.controller.Library`.
Props joemcgill, westonruter.
See #21819.
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