Put back participants and submissions counters

[Didayolo]

Counters removed to solve the performance problem:

• URGENT - removed count from competition query to fix the codabench #1645

• We need to re-implement it in an efficient way.

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• We need also to put back the "entries" counter on the leaderboard:
• Removed num_entries becasue the count was slowing down the platform #1673

[Didayolo]

Duplicate of #1327