GNDT Changes

I've made some overdue changes to the GNDT login system; encrypting the password, making the logged-in status a bit clearer, using separate cookies for separate Nomics (so that you can log in and out of each independently, with different passwords - sorry if this has been annoying the overlapping Sparta Nomic players, I hadn't realised it was happening), and changing the default GNDT-target from "blank" to "yourself", which stops the annoying "must remember to select a target player if you want to just add a comment" bug.

The cookies should migrate themselves into encrypted ones without you noticing, but let me know if I've inadvertently broken anything.