Output external images with unknown dimensions in AMP pages

Output external images with unknown dimensions in AMP pages

The `layout="fill"` attribute allows to embed an `<amp-img>` with unknown
dimensions. This comes with the drawback that the image will stretch as
far as possible, not maintaining the aspect-ratio and taking up the whole
page.

To solve this a container with known dimensions needs to be put around it
and the `<img>` within needs to have the `object-fit: contain` property.
This property will contain the image within the container while maintaining
aspect ratio at the drawback that the empty space of the container will be
filled with void.

This container was selected to be a 16:9 aspect ratio, with a size of
384×216 Pixels (one fifth of Full HD in each dimension), hopefully
minimizing the void on common image formats.

The resulting AMP was checked against the AMP validator and passed.

Fixes #3264