Keyboard accessibility for the media modal:

* Arrow keys navigate between items in the grid.
* Transfer focus into the panel when selecting a tab along the side.
* Transfer focus into the details sidebar when selecting an item and vice versa.
* Set initial focus on the close button so that it is visible.

props celloexpressions, lessbloat, ericlewis. fixes #25100, #25101, #28704. see #23560.

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