Add Mojang Ownership linking

[alikindsys]

Database

This would likely require adding an mojang_uuid: Optional<Uuid> field to the User object and its respective table.

Integration

We would need to check if an username is a valid mojang account.

Design Considerations

It might be beneficial to cache all player names which we previously fetched as valid minecraft accounts, and their respective uuids somewhere, and provide our own way of tracking name changes based on that. (See #11)