Support for inter-site tripcodes.

[skord]

The problem with secure tripcodes is that they are based on a site specific salt, and therefore, aren't really useful anywhere else. An API should be provided to allow users at foreign sites to enter a site specific tripcode and have the local site provide this on demand. Something like skord##usersalt put in locally should be accessible in a remote form like skord##usersalt@skord.org