Improve get next ungraded request
In terms of making the server the source of truth for getting next ungraded, we should check what submissions are ungraded before going to the next ungraded.
From experience in large exams, the updateAllSubmissions request becomes very slow: the request will serve multiple megabytes of data, and also take a while to compute. In our exams, up to 3 seconds.
Updating all submissions on every feedback option toggle, is too often. This should not call updateAllSubmissions, but updateSubmissions.
Updating all submissions when doing next/previous ungraded is also too often, because it takes too long.
updateAllSubmissions should therefor not be used in the grading page at all. The only reason we call this in the grading page is to update what submissions are currently marked as graded.
We should implement a request that only gets {submissionID: id, graded_by: id, graded_at: timestamp}. This is enough information to find what the next ungraded would be.