added more hardening against malicious messages.

Ignore-this: 7b182d1f52278b940406ad2d28bff536

modified the session, script, chat, and app editor to only accept messages that are
encrypted with symmetric encryption, meaning that the person communicating must have
authenticated and connected via the user service.

darcs-hash:20140223184150-c21fe-4d5323b6887c0af40376f2a72e4b83778fdb7a38