Evaluations: ask to preserve score items when selecting another submission manually

[pdawyndt]

In an evaluation, when selecting another submission, Dodona deletes all comments (annotations) and scores assigned to score items. It warns about the removal of comments, but not on the removal of score items.

We could make these warnings conditional (if no comments were given and no scores were assigned then the warning is not really needed). Seems hard to migrate comments from one submission to another (since they are atteched to lines in a submission), but when score have already been assigned, it would be better to ask the user whether or not these scores should be retained. Users can easily scores themselves (even in bulk) if needed, but keeping scores would definitely be an added value (e.g. when switching to another submission with only minor differences).