Removed auto-register and made players require to set a password

[ThomasWega]

This was done to fix a crucial issue I encountered. Cracked players with nicknames that are already existing on a premium user were not able to join at all and always gotten invalid session message. This was caused by this specific part

This led me to realize there are only two options at this point

1. now allow any cracked user with premium username to join
2. remove auto-register and make everyone register with a password. Premium users can then do /premium and enable auto-login. If cracked users do this they will lock themselves and it will need to be resolved by an admin though.

I am much more comfortable with option number 2 and that is what this PR accomplishes.

The reason this is a draft is because I feel like this should be a switch as is with other auth plugins. I'm looking for feedback how to implement that nicely

[Navio1430]

Invalid session error is a default Minecraft behavior. The error happens when a cracked player wants to join with a premium username (and their session is set to premium, because of the nickname).

NavAuth could have a toggle for the autologin or support some kind of an auto-migration (combined with descriptive invalid session error).

[ThomasWega]

Yes, precisely my point. I'd be happy to implement this sort of functionality. The code is not prepared for this from what I've seen though, so I'd like some guidance on how to approach it nicely so it doesn't bloat up the code