Add dataset images

[ivan-aksamentov]

Add optional images to datasets.

Sibling PR in software: https://github.com/nextstrain/nextclade/pull/1262

The image.png or image.json file can be added to dataset directory along with the Image field in pathogen.json:

"image": {
    "path": "image.png",
    "source": "nextstrain.org"
},

This allows Nextclade Web to use these images in the UI.

In this PR I copied some of the images from this git version in nextstrain.org: https://github.com/nextstrain/nextstrain.org/tree/84b5a589b5e819352b52ee6104f1e25fd77bef12/static-site/static/splash_images