Add message about CC0 License to Editor

This is meant to resolve https://github.com/hypothesis/vision/issues/90. It now looks like so:

![Editor with CC0 License](https://cloud.githubusercontent.com/assets/521978/5405934/c83d0f64-8160-11e4-9ce2-0daafb1cf46c.png)

The link points to: http://creativecommons.org/publicdomain/zero/1.0/

After thinking about it for a while I came to the conclusion that it would be good to get more feedback on the license we've chosen, and making the license clearer is a good way to get that feedback (because as @BigBlueHat insists: *no one reads the Terms of Service*). If need be, or if it confuses people in user tests, we can easily remove this.