Disable the member topics feature for now
The topics feature is currently non-functional (it calls non-existant methods on the MailingList object), and a proper fix would require a schema change.
This merge request just disables the feature, since mailing-lists imported from 2.1 with topics enabled just cause tracebacks currently.
I think it's better to disable it this way rather than forcing the import to unset the topics_enabled
flag, to remember which lists wanted the topics feature.