Delegate Document Approvals

Delegate document approvals the same way we can delegate issue and task approvals.

Workers and Reviewers should be able to delegate their document approvals if they know in advance they will be OOO, traveling, etc.

We use the document approval process to route proofs for approval, but I think this functionality would be beneficial for proof approvals as well, since we eventually hope to use Workfront proof once we are satisfied with the progress of the integration.