Switch back to usering a subquery to eliminate superadmin users

In the end I think this is better because its more obvious what we're
doing. Its still not nice having the subquery, especially with table
prefixing in there. However I'm not happy repurposing access_level
for this.
Merge pull request #4 from rjmackay/1080

Switch back to usering a subquery to eliminate superadmin users