Review sign offs

Functionality to facilitate reviewer sign-offs should be made available in Concourse. Currently, reviewers are notified if there is a deliverable tagged for their review and they can tag the deliverable as "Completed" when they complete the review. However, integrity of the sign-off is not maintained as anyone can update the status to "Completed" (this may happen if the review was done via a call). Moreover, there is no history of who updated the status and the date it was updated.