AnonymousUser self register

[KhaledBousrih]

Currently any user can be self registered to concrete datastore.

In some cases, we don't want that any user can be self-registered, so this must be enabled/disabled by a settings ENABLE_USERS_SELF_REGISTER default to True in order to keep the existant:

If ENABLE_USERS_SELF_REGISTER is False, we must fail any regiter request that have been performed by an anonymous user (not authenticated) => 400_BAD_REQUEST with the data

{
    "message": "Self register is not allowed",
    "_errors": ["NOT_ALLOWED_TO_SELF_REGISTER"]
}