Batch of changes that creates a PhantomNet portal branding.
Also includes some PhantomNet-specific restrictions (e.g. only allows use of the main Utah Emulab testbed aggregate). This excercise stretched the limits of what we can reasonably do before introducing real per-testbed branding/policy mechanisms to the php/web front-end. My changes ain't exactly pretty... Please take care when adding any testbed-specific changes to the code. There are three flavors now to consider in the logic.