Media: improve flows for cropping on attachment details screen.

On the "Attachment Details" screen:
* The crop button is always 'enabled'.
* Clicking the crop button with no selection selects the entire image.
* Clicking the crop button with the entire image selected doesn't do anything.
* Clicking the crop button with a selection crops as expected.

Props sonjanyc, afercia, mikeschroder.
Fixes #30155.

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