Tasks completed before this change have no doneAt value; the drawer now
always renders the Completed at row for completed tasks and falls back
to a dash instead of hiding the row.
Add DoneAt to the download task model: set when a task finishes and
cleared when it is downloaded again. The value flows through the API
model and task record, and is shown as "Completed at" in the task
drawer info tab next to the creation time.
Levi
Levi