Media grid, round 2. Expect much more to come.

* Instead of a sidebar for details, utilize a modal. The modal experience allows for a larger preview, editing images, audio/video previews, and previous/next navigation, like the theme browser. Think of it as an attachment browser.
* Show some details in the grid view to more easily distinguish items.

props ericlewis, wonderboymusic, JerrySarcastic. see #24716.

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