Approvers in the merge requests list (Web and API)

Description

• Applies to both project merge request list view and group merge request list view.
• Applies to all three tabs in the merge request list view, i.e. open, merged, closed.
• Update the API to return this same information.

• View who has approved a merge request, and who still needs to approve using a hover UI.

Mockups

• When the number of approvals has been met, show only those approvers

• When the number of approvals has not been met yet, show who has approved, and anybody else that can approve.

• How it looks zoomed out.