add doneAt date to setStatus mutation

[switzerb]

This adds an optional doneAt Date scalar to the setStatus mutation on the planner. This piece of data will allow us to "complete" a recipe on a day that is not today. Which, as it turns out, is kind of a thing for me.

To test out the mutation:

mutation setStatus($id: ID!, $status: PlanItemStatus!, $doneAt: Date) {
  planner {
    setStatus(id: $id, status: $status, doneAt: $doneAt) {
      id
    }
  }
}