Adds a new leaderboard feature between a user and their friends

[guimann2015]

Adds a new leaderboard feature that allows users to compare their ratings
against their friends across different game variants.

This implementation includes:

• Privacy controls to ensure users can only view their own friends' rankings,
• Support for singular/plural translations (e.g., "your only friend" vs "your X friends"),
• Multilingual support (EN, PT, ES, FR, BR),
• UI integration with existing following/friends system,
• New route (/@/:username/following/leaderboard)

This is the new button in the friends page:

This is the friends leaderboard for a user with more than one friend:

This is the friends leaderboard for a user with no friends:

Co-authored-by: Francisco Guerreiro francisco.franca.guerreiro@tecnico.ulisboa.pt
@franciscofguerreiro

[franciscofguerreiro]

@ornicar would appreciate your review when you have a moment. Thanks!

[ornicar]

Before going into the code details, I have a few reservations about the viability of the feature.

The ranking collection is designed for site-wide leaderboards. It only contains players who have played a ranked game in a given category during the last 7 days. The point of this is to prevent a player from sitting on a peak rating for too long, and also to reduce the amount of documents.
It works well for a site leaderboard, not so much for friends. Players will wonder why some are missing from the leaderboards.

Another thing is that people can (and do) follow 400 other players. Displaying that page can then be quite costly.