Display applied waiver/license; offer CC0 anytime.

Fixes #369. As requested, the UI can only apply the CC0 waiver to a
study that has no waiver or license already. This is a "ratchet", and
CC0 can't be revoked here after it's been applied. (The only way out is
to cancel or close the curation session without saving changes, or to
modify the NexSON in the docstore.