Serve images as webP

[robbevp]

If we can create variants for an image, we should also create webP variants for the different sizes.

We might also consider just serving webP variants, since only safari and IE do not support it yet.

[chvp]

Why?

[robbevp]

This will reduce file size, thus reducing load times, cache size (when Rails 6.1 fixes the caching of variants), and the amount of data transfered