Sort merged merge requests on merged_at attribute

Problem to solve

When going through the merged merge requests, there's no ability to sort based on when the MR was merged. Which makes the order or the list differ from the Git history.

Intended users

• Parker (Product Manager)
• Delaney (Development Team Lead)
• Sasha (Software Developer)
• Devon (DevOps Engineer)
• Rachel (Release Manager)

User experience goal

Allow the merged merge request list reflect the Git history to understand for example which order commits are deployed in.

/cc @andr3