Replace via.placeholder.com with placehold.co

[runephilosof-abtion]

Fixes #3055

Checklist

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[runephilosof-abtion]

Oops. I did not check the differences in the APIs.
placehold.co does not support just specifying a background color.
If you specify one, you must specify the other.

What color did via.placeholder.com use when only specifying the background color?
I assumed the defaults were black on white and that specifying one would only change that one. So I retained that functionality.

Will push the new commit soon ✔️