Add a setting to restrict organization creation right to super-user (admin) only

[fallen]

This could be used in addition with the "only organizations can publish petitions", see #227

This setting would change the behavior of the instance as follow:

• Only admin (Django super-user) could create organizations
• This setting could be set and unset at will, with no irreversible effect

That would prevent users to circumvent #227 by creating ad-hoc organizations to publish petitions on their own names.