Add new roles for MR review process

I would like to have people in the list of current “reviewers” who cannot approve the review, but may participate in the review. The roles needed are at least Approver, Reviewer, Observer & Author.

• Author already exists
• An observer can only see the review, but cannot make comments (Silent Reviewer).
• A reviewer can add comments and vote, but cannot approve.
• Approver already exists

ZD Reference: https://gitlab.zendesk.com/agent/tickets/37434