Fix BestModeStrategy to exclude soft-deleted and non-finished submissions

[Didayolo]

A brief description of the purpose of the changes contained in this PR.

This PR ensures that BestModeStrategy, used for publishing results on the leaderboard by forcing the best result, only picks up submissions that have finished (and may have a score attached) and are not soft deleted.

Issues this PR resolves

Fixes #2044
Fixes #1284 (failed submissions are now discarded, blocking n/a scores associated with failed submissions).
Fixes #1840
Fixes #2057

A checklist for hand testing

• Create a leaderboard with "Force best" strategy
• Create a submission, it should pop up on the leaderboard, as it's the only one
• Soft delete it, it should disappear from the leaderboard but still count towards the daily/per-person limit
• Reupload the same submission (or upload a submission with a worse score), this should now be present on the leaderboard

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge

[Didayolo]

@ihsaan-ullah @ObadaS : It looks like submissions are not working on the test server AND CircleCI tests do not pass.

How to validate PR in this situation?