demo account for non-MIT users

[price]

A public-facing web page, #78, would be especially effective if it comes with the same first-five-minutes of "wow, this is cool" in creating a VM and getting a console that we've found so powerful for convincing supporters within MIT.

To accomplish this, we might consider having one "locker"/owner for unauthenticated web users. Of course we have to worry extra about security and abuse. To prevent spamming, we might ban this demo user's VMs from using the network; this would still allow them to use the cloning autoinstaller, or boot a livecd, which is plenty for the "wow, cool" we're aiming for. To mitigate the risk of guest->host exploits in Xen, we might run such a user on a separate test installation of the entire system, with its own database and storage on local disk; once we've set up a new compute server in the course of #74, this may not be too difficult to do on a random box, and is probably a good thing to have in itself.