Extend contribution with ERC721 metadata

[raucao]

Some properties, like e.g. "description" are already used in our schema, but we might want to change the meaning of the description, based on how ERC721 tokens would render in a client that implements the standard. In any case, we can also add a token name and image URL too.

Here's the metadata schema, according to EIP 721:

{
    "title": "Asset Metadata",
    "type": "object",
    "properties": {
        "name": {
            "type": "string",
            "description": "Identifies the asset to which this NFT represents",
        },
        "description": {
            "type": "string",
            "description": "Describes the asset to which this NFT represents",
        },
        "image": {
            "type": "string",
            "description": "A URI pointing to a resource with mime type image/* representing the asset to which this NFT represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive.",
        }
    }
}