WIP: allow skipping notifications when confirming

This is a probably very naive attempt at fixing #510 (closed) by allowing API callers to not send confirmation messages when subscribing users.

The API, frankly, seems a bit wrong, as "notify" is a bit too generic: notifications would be still sent to list admin for moderated lists, for example. I'm not sure whether those should be censored as well or the API changed.

I'm not even sure it's the right place to implement this. But I believe it works. I still need to test this, but the idea is to delegate the confirmation mail notification to the caller instead of letting mailman do it. This makes #541 (closed) moot as well since the caller can translate the message or configure it as needed.

An alternative way to do this might be the admin_immed_notify list-specific setting, but I'm not sure it's the right place (list-wide) to implement such a setting.

Thanks for any input.