Support 'use original image' for header images

[drzax]

Also adds a bunch more type definitions.

@AshKyd — I'd be interested to know how this compares to the image part of https://github.com/abcnews/interactive-hero-images

I think it makes reasonable assumptions about what to do at different viewport sizes, but there is definitely some compromise.

[AshKyd]

I can't really tell what's going on here, maybe we could go through it next week?

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[drzax]

Thanks for the convo on this yesterday @AshKyd, it led to a number of small but good improvements. I'm keen to merge this asap, but if you feel like having a quick look, I'd welcome the additional eyes.