API user creation behing a toggle

I've added a toggle feature (in internal settings) so that user api creation can be disabled while form registration still can be enabled.
Also, the /api/user endpoint shouldn't require authentication. Even if we check the authentication when sending a GET request, to retrieve current user information.

I've moved all the internal settings definition to config to avoid duplicated place to define them.
I don't know why we didn't did that earlier.

app/config/security.yml

@@ -56,6 +56,7 @@ security:
     access_control:
         - { path: ^/api/doc, roles: IS_AUTHENTICATED_ANONYMOUSLY }
         - { path: ^/api/version, roles: IS_AUTHENTICATED_ANONYMOUSLY }
+        - { path: ^/api/user, roles: IS_AUTHENTICATED_ANONYMOUSLY }
         - { path: ^/login, roles: IS_AUTHENTICATED_ANONYMOUSLY }
         - { path: ^/register, role: IS_AUTHENTICATED_ANONYMOUSLY }
         - { path: ^/resetting, role: IS_AUTHENTICATED_ANONYMOUSLY }