Leaderboard: Add userlist as "Group entries by" option

When selecting group entries by in leaderboard widget (advanced settings), we are presented with two options: grouping them by users or teams. However, in numerous use cases, it becomes more logical to group the entries by userlists.

By grouping entries based on userlists, we can cater to specific scenarios where collaboration and information sharing revolve around predefined groups of users. Userlists are widely used in other widgets & visibility settings. It's also possible to create dynamic userlists by profile fields.