GScan should validate `image_sizes` config/use in a theme

[allouis]

I guess we'd want to:

• validate that width/height are numbers?
• validate any use of the img_url helper either omits the size or the size exists.

@JohnONolan I've moved this out from so that we can close it TryGhost/Ghost#10181 - you might be able to add some more things that we could/should validate?