MR screens carry review, check and resolution times !37

Parity with krz/gitbay!137, which added the fields to mr show: created_at per review, updated_at and duration per check, and merged_at/merged_by, closed_at/closed_by on the merge request.

• Reviews and checks show when they last said something, as a date and time rather than "3 days ago" — the point is to read it, not to know it was recent.
• A ci/<job> check shows how long its build ran. Checks posted through status set have no build and show only the time.
• A merged or closed MR shows who resolved it and when, under the header.

Every added field is optional, so an instance running a server without them decodes and renders exactly as before.