Refactor User and TrustLevel a bit

* rename `User#password_required` to `User#password_required!`
* emails with "i" @ something are a special case as well
* get rid of `self.` and returns where possible
* prefer "unless a" instead of "if !a"
* `unread_notifications` without manually iterating
* introduce `User#moderator?`
* introduce `TrustLevel#valid_key?`, `TrustLevel#compare`, and `TrustLevel#level_key`