Allow disabling "New User notification" without removing SMTP Server config.

[einhirn]

In https://github.com/bigbluebutton/greenlight/blob/9d1b0249a4e14bec39e7f8dad2b0952fa1ba880f/app/controllers/external_controller.rb#L58 GLv3 sends a "new user" notification as soon as an SMTP server is configured - this should be configurable or at least check whether any further action is required after new user creation. Currently those messages are just spam for us.

[farhatahmad]

There is a role permission that you can turn off if you don't want the emails