PrintJob  
  name          Text
  filename      FilePath
  file          ByteString      -- stores plain pdf; otherwise use FileContentReference Maybe
  created       UTCTime
  acknowledged  UTCTime Maybe
  recipient     UserId Maybe    -- optional as some letters may contain just an address
  sender        UserId Maybe    -- senders and associations are optional
  course        CourseId Maybe        OnDeleteCascade OnUpdateCascade
  qualification QualificationId Maybe OnDeleteCascade OnUpdateCascade
  deriving Generic