Player team entries are stored as player names

Whenever a player gets added to a team, their name gets stored in the Players key, instead of their UUID.

This will cause issues in two scenarios:

1. If a player is added to a team, quits the game, changes their name and then rejoins the server, they'll be removed from the team.
2. If in the future longer names are supported, a player could name themselves as a UUID and cause potential issues.