Peer Closing for Multiple Assignee issues

Description

Since https://gitlab.com/gitlab-org/gitlab-ce/milestones/35 we have the Multiple assignee feature for issues. That's great. What if we can have multiple closers?

Proposal

By this i mean that an issue (mainly intended for multiple assigneed issues) before get closed is something like peer reviewed by the other assignee.

Links / references

This idea is not new on the World Wide Web. I got it from stackoverflow's peer review of edits feature.

Documentation blurb

1. Issues get closed by someone. We trust him/her close. Or not?
2. A peer_closing status of the issue means that an issue is being closed but another user should confirm the close of the issue.
3. From an issue with 2 assignee a and b when a closes the issue b (or b2 or b3) has to confirm it. Then the issue get closed. during this time the issue is in a peer_closing status;