Show "Merged at" in the list of merge requests

Problem to solve

When looking at merged MRs what you often want to know is 1) when a MR was merged and 2) the order of merged MRs. Currently this is very difficult; every merged MR shows a giant "MERGED" label but no information when it was merged. You can see and sort on the "Last updated" property, but this can easily change after the MR was merged (e.g. by mentioning it in a commit message).

User experience goal

The user should be able to see when a MR was merged and to sort on this timestamp.