add tab challenge leaderboard

[maikia]

Description

It would be of interest to have combined leaderboard from all the events run for one challenge.
This would be visible to all the users but the links to the actual solutions should be deactivated.
If one user takes part in more than one event only his/her best solution score should be displayed

Steps/Code to Reproduce

Expected Results

Actual Results

Versions

[rth]

This would be nice. In the workshops I did, people keept asking if their scores were good or not compared to others in the beginning of the challenge.

[agramfort]

sounds useful indeed !

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