Skatepark: Apply duotone effect to Featured Images by default

[kjellr]

Related to #4329 and #4406, but specifically for featured images.

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Skatepark's featured images should all have the duotone feature applied by default:

For FSE users should be able to turn this on and off by editing a template (there's a pending issue in the Gutenberg repo to add this functionality to the Featured Image block). We'd theoretically need to turn this into a customizer setting for folks without FSE as well.