Make movement server authoritative

[stilnat]

Summary

Movement happens completely client side currently. It's an open door to all kind of cheats. We need the server to check if movements are legit, and to handle the position of the players. We'll need client-side prediction to make movement smooth on the client.

a small wikipedia introduction to client-side prediction.

Some docs about authority and Mirror, our networking library

@Ryan089 @joaoburatto If you have something to add on the technical side of things, that'll be great !