Currently the handling of auto_join_rooms only works when a user
registers itself via public register api. Registrations via
registration_shared_secret and ModuleApi do not work
This auto_joins the users in the registration handler which enables
the auto join feature for all 3 registration paths.
This is related to issue #2725
Signed-Off-by: Matthias Kesler <krombel@krombel.de>