[Submission View] Display how many points a contestant gained by submitting this solution

In the current view we only see the raw number of points. It would be way nicer to also display how this submission affected the scoreboard.

Example:

- A submitted solution for problem X, got 0 points (gained points: 0)
- A resubmitted X, got 20 points (gained points: 20)
- A resubmitted X, got 20 points (gained points: 0)
- A resubmitted X, got 100 points (gained points: 80)