Move from Apache-2.0 to CC-BY-4.0 License

The Apache-2.0 license isn't really optimized around non-source-code assets, so we'd like to move to the Creative Commons Attribution license instead, so they can still be used freely, but it's much more concise and appropriate for an image repository.

This PR is to encapsulate the changes to the repo as a result of this decision, and to make clear that consent was granted by all copyright holders involved.