Cancellations should have a note attached

Sometimes jobs have a valid reason to be cancelled (and stay that way) -- a recent example being https://github.com/NixOS/nixpkgs/issues/91145#issuecomment-646975082. As it stands now, there is no way to let people know why this job was cancelled, aside from being mentioned on one of our many IRC channels, or in an issue somewhere.

It would be extremely useful to have some sort of accountability for cancellations and restarts, such that attempting to restart a cancelled job will display a note written by the cancelling user and advise against restarting the job.

Inspired by this discussion on #nixos-dev: https://logs.nix.samueldr.com/nixos-dev/2020-06-20#1592677835-1592678545

Semi-related: Maybe showing "cancelled by [username]" and "restarted by [username]" somewhere for logged-in users would be a nice addition. This would allow someone to reach out to the user who cancelled a job and ask if it is necessary anymore (if they forgot to include an issue link).

Start by locating the job cancellation and restart flows; the payload names no files, tests, or entry points. Done means a cancelled job can retain a cancelling user's note, show it when restart is attempted, and expose cancellation and restart attribution as appropriate.