confirmation mail Subject should be translatable, if not templatable

in #510 (comment 138147180), I have noticed that the confirm message sent for subscribe and unsubscribe requests is untranslatable. it's hardcoded in src/mailman/app/subscriptions.py as such:

def _handle_confirmation_needed_events(event, template_name):
    subject = 'confirm {}'.format(event.token)

This seems strange to me: there's no way to translate or modify this in any way, from the user or even the admin's perspective.

I think this should at least be translatable, like, for example, the welcome message:

        _('Welcome to the "$mlist.display_name" mailing list${digmode}'),

The unsubscribe confirmation (which follows an unsubscribe confirmation message of course) is also translatable:

        _('You have been unsubscribed from the $mlist.display_name '
          'mailing list'),

It seems strange that the first confirmation mail subject is translatable but not the second one.

Furthermore, it seems to me we should allow list admins to change those subjects as well. After all, if they can change the body, why can't they change the subject? It's the most visible part of the email, yet it's out of control of the user.

I would suggest we could simply take the first line of the template and use that as a subject. It's a breaking change, but from what I can tell, many templates are already designed that way. Otherwise this would complicate template maintenance significantly...

But I feel that, at the very least, the subject should be translatable.