Store multiple graders and approvers per solution
Currently the last person who changed the state of a solution is seen as the grader of that problem. We should extend this such that we store exactly who helped grading and who approved a solution. This idea started in this discussion on Mattermost. Another result of this change is a better estimate for the total grading time.
This can be done by creating a new table storing (solution_id, feedback_id, grader_id, state, timestamp), where an entry is added on each feedback option toggle/approval. Then we can show in the UI who helped grading a solution, who approved it, etc. This can also be used to show the full history of the grading process for a solution. Another benefit is (in the far future) the ability to roll the grading back to a specific timestamp.