Replace the current `wp.media.gallery` instance in `media-editor.js` with one...

Replace the current `wp.media.gallery` instance in `media-editor.js` with one that leverages `wp.media.collection`.

See #26631.


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git-svn-id: https://core.svn.wordpress.org/trunk@27070 1a063a9b-81f0-0310-95a4-ce76da25c4cd