Creative Commons Licensing for edX

This PR adds Creative Commons Licensing to the Open edX Platform.

Review tickets
T&L https://openedx.atlassian.net/browse/TNL-195
UX https://openedx.atlassian.net/browse/UX-948
You don't seem to be using these attributes anywhere. Why do you have them here? (Note that this is exactly the right place to store data about the license of an asset -- I'm just confused why you're not actually using it.)