Add native lazy-loading support

[danoc]

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[danoc]

Sorry for the noise here. This is good for review!

[danoc]

I'm starting to realize this won't be as impactful as I hoped.

Two issues:

• Chrome is very eager about when it starts to load the image. There's some discussion about improving this.
• sizes gets calculated with JavaScript. If we defer to the browser for lazy-loading, the browser will assume the image is 100% width, often choosing the wrong size in a set of responsive images. There's discussion about improving this as well.

This CR does improve performance on pages that have heavy use of carousels. But we can fix the carousel bug separately and see performance improvements without these downsides.