Media Grid: on page load with `?item=` in the URL, if the model is not in the library: fetch it. Defer the triggering of `edit:attachment` until the model has been fetched.

Fixes #29052.

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git-svn-id: http://core.svn.wordpress.org/trunk@29301 1a063a9b-81f0-0310-95a4-ce76da25c4cd