Shorten processed image filenames

[bep]

I'm doing some consolidation in the hashing department that will change the filenames of generated images, so this would be a good time to do this.

A resized sunset.jpg will typically become:

sunset_hu59e56ffff1bc1d8d122b1403d34e039f_90587_400x200_fill_q75_box_center.jpg

My motivation above was probably to be able to see the process steps in the name, but that info gets lost in the more complex setups anyhow, and is hardly worth the ... verbosity.

I suggest that we instead create something that would look like:

sunset_hu3730316029470468128.jpg

[github-actions]

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