Add support for package and element icons and screenshots

[justinfagnani]

For catalogs we'll want to display icons for packages and elements. I'm not sure what formats are best to use - we should probably look to PWA and extension manifest formats to see how they do it.

[justinfagnani]

See https://developer.mozilla.org/en-US/docs/Web/Manifest/icons and https://developer.mozilla.org/en-US/docs/Web/Manifest/screenshots