Avoid generating loops on bounced admin notifications
At the moment, we notify admins about every bounced message to the list. The sender of such an admin notifications is the list itself. Which means that when the notification bounces, it will bounce back to the list, that triggers another notification to the admins and we have a glorious loop.
In schleuder 2 we avoided that by setting the sender to the superadmin https://git.immerda.ch/schleuder/tree/lib/schleuder/log/outputter/emailoutputter.rb#n7 / https://git.immerda.ch/schleuder/tree/lib/schleuder/log/outputter/emailoutputter.rb#n103
To void loops on bounced notification emails, we should also set the sender to the superadmin address.