add `needs_author`, `requires_express_permission_for_publication` and `url`

update LicenseSerializer and license fixture

- replace LicenseType with License model, update fixture, serializers, views, and urls

Closes: #167

This reverts commit 753af8b4.

Closes: #151

- replace cc0: No Rights Reserved with pd: Public Domain
- prefix the Creative Commons License names with `cc-`
- add gfdl: GNU Free Documentation License