Refactor login packet handler for more flexible auth

[yungcomputerchair]

This PR enables auth cookies to be used simultaneously with plaintext paasswords sent in the cookie authID field.

• Hoist a bunch of checks from the login packet handler into helper functions.
• Rename the LoginType enum to AuthMethod and distinguish it from the iLoginType packet field (see comment in code for why these should be decoupled).
• If the provided token does not pass the cookie check and password auth is enabled, treat it as a plaintext password and authenticate if it is correct.