Inconsistent graded_at and graded_by
Problems can currently have a graded_at, but not a graded_by. This results in a nullpointer in grade.jsx:349.
This happens (sometimes) when first a requests removes the last feedback option, and very quickly after a request adds a new option, resetting it to graded. Something goes wrong here, and results in a state where
graded_at != null and graded_by == null