Multi-task not working with Force Last submission rule

[Didayolo]

When you have more than one task and leaderboard rule is Force Last or Force Last Multiple:

• You see n/a because one of the child submission is on the leaderboard and the other one shows n/a.
• Also, on "My Submissions" tab, no submission is marked as "on the leaderboard". Failed submissions are marked as Finished.

We should NOT show the child submission on the leaderboard. Only the parent submission should be shown with scores from child submissions.

It looks like when the rule is applied, it is applied to submissions regardless of if they are parent or child.

Reported in:

• Score n/a for one of the subtasks when submitting for both subtasks at once. #1570
• Wrong leaderboard ordering and n/a scores discussion #1540