Add CC0 as a license

This commit merely adds a license page for CC0 as /licenses/cc0.

Per the discussions at https://github.com/project-open-data/project-open-data.github.io/pull/135 and https://github.com/cfpb/qu/pull/94, there are no substantive differences between CC0 and Unlicense. Practically speaking they are interchangeable. But CC0 has a strong community and is being strongly pushed within government, so it at least deserves a mention.