API user creation behing a toggle

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.