Customizer: Improve handling of crops in the media library.

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.

Built from https://develop.svn.wordpress.org/trunk@41937


git-svn-id: https://core.svn.wordpress.org/trunk@41771 1a063a9b-81f0-0310-95a4-ce76da25c4cd