Leaderboard stages don't have their own URLs

[romovpa]

The problem:

• I go to the leaderboard
• choose the "3 stage"
• click to the user image -> get to the profile page
• click on browser's back button -> return to the first tab of the leaderboard, not the "3 stage"

I propose to make separate URL for each leaderboard tab
https://ods.ai/competitions/sberbank-covid19-forecast/leaderboard/3-stage