Add option on document_encrypt to not encrypt to current user

[ernieturner]

New boolean option, (grant_to_author default true) to DocumentEncryptOps that allows callers to not encrypt to the user performing the operation, but only to those users/groups in the provided grant list. Validation must then be performed to verify that at least one user/group is being encrypted to, that is, this flag cannot be turned off if the list of grants is empty.