Block circle & project creation to non admin users

Rémi Arnaud, from La Briqueterie, requested such a setting.

the biggest point will be having an admin interface to control this setting. But if it's just a setting to change in the configuration file (allow_circle_creation_by_users: True) there is just one line of code to change the permissions in Django, and some more work to hide the creation button on the client side if add permission is not there.