I realize the API is built like this, but shouldn't self-assessment be based off of the `submission_uuid`, and not the student_item? If we passed just `submission_uuid` into the self API, it should be able to grab the info it needs from submissions. I'm more concerned in the longer term because it is plausible that we might have multiple submissions for a problem at some point, in which case this is associating the assessment with the wrong thing.