Add setting to prevent creation of new users

There should be an option to prevent the creation of new users, such that existing users can login via the Helmholtz AAI (eventually via the HELMHOLTZ_UPDATE_USERNAME setting), but not signup through the AAI. This is helpful if the Website does have a custom sign up-form for instance (e.g. the clm-community at https://hcdc.hereon.de/clm-community)