Our Haystack signal processor listens for newly-saved users and, if

search indexing is enabled, adds them to the index. The problem is,

the act of checking if they're enabled (looking up a

SiteConfiguration) can fail when creating a new database with a

superuser for the first time.
The reason this happens is that django.contrib.auth will prompt the

admin to create a superuser in response to the post_syncdb or

post_migrate signal the first time that the User model is added to

the database. This occurs before django.contrib.sites has a chance to

create its Site object. Normally that's fine, but our signal processor

is getting in the way by operating on that new superuser and accessing

the not-yet-created Site.
This wasn't a problem in the past because we used to shell out to the

syncdb command, which never invoked the code that registers our signal

processor. A recent change removed that in favor of directly interfacing

with the Django Evolution Evolver class, meaning that the process is

now taking place in an environment where the signal processor is

registered.
This change updates the signal processor to check whether we have the

right entries in the database before handling each signal. This fixes

prepare-dev.py.

Unit tests pass.
Successfully ran prepare-dev.py for the first time with a superuser,

and saw that the database populated correctly.