Use user mentions in CodeyCoin Leaderboard instead of usernames

[Fan-Yang-284]

Currently, the CodeyCoin leaderboard uses usernames to display as raw strings, but we should use user mentions (<@id>) since they look nicer. Note that since the leaderboard is displayed in an embed, the leaders won't be pinged.

See original suggestion and more information here:
https://discord.com/channels/667823274201448469/762882537692332064/1228578319998521375
https://discord.com/channels/667823274201448469/1007014760761540679/1228578890083991592