Display and allow sorting by the merge timestamp for CRs, not just the creation timestamp

[JaniceManwiller]

The merged CRs list is always displayed in descending order based on the creation timestamp.

In most cases CRs are merged in the same order that they are created, but not always.

There is no option to sort the merged CRs based on their merge timestamp. It's very confusing if a CR happened to malinger a bit because of outside issues and it ends up being third or fourth in the list in spite of it having been merged most recently.

There is no indication of the merge timestamp either in the merged CR list or the CR overview. It's always only the creation timestamp.