Restrict user creation

User creation is now unlimited, and all users are sort of admins. I probably won't want anyone to be able to change the settings for me websites, so I'd like a mechanism either to block more users from creating accounts, or to have a system to differentiate admin and regular users.

Anyway it's fine that once freshly setup Commento allows one user to register, and that this user will become the first admin. A lot of software behave this way.