Prefer CC0 for artwork

[Calinou]

It would be better if artwork used the CC0 1.0 Universal waiver, rather than the MIT license which is a software license.

The upsides of it are:

• it's a license suitable for everything, including artwork and databases,
• it is compatible with all GPL versions (like MIT).

If CC0 is not possible for you, you can also choose CC BY 4.0 which has similar upsides to CC0.