Allow disabling the guest feature

In case the server that you specified in the Chatrix block doesn't allow guest registrations, the message of #211 appears and you cannot do anything.

In addition to fixing #211, there should also be an option to not use the guest feature even if the server supports it.

Start by reading issue #211 and tracing the Chatrix block's guest-registration flow and configuration entry point. Determine how the guest feature is selected and how the server rejection is surfaced; done means users can disable guest use and are not blocked by unsupported guest registrations.

Written by the indexing model from the issue text.