Show overall user statistics (leader list)

[joergreichert]

Current state: There is no (public) available information, which users did water how much in current watering season.

Motivation: I want to know who are the most active / best tree waterers in Leipzig, because this motivates me to water more, when I by this can climb up the lead list.

Future state: There is an extra sidebar icon, where I can open the sidebar that shows the current leader list of the users watered the most liters. There should be an opt-in checkbox in the user profile, to allow or disallow appearing in the leader list with either the username or pseudomized random name (default is disallow).