Should be able to do double opt-in via profile form addition to group

Right now, if a profile adds a user to a mail group, it does so with the status "Added by admin", which means no opt-in mail is sent.

We need to be able to have a user subscribe to a group via a profile form (which also collects other information, so the standard mail subscription form won't work for this), and then receive an opt-in message. The status of the user should remain pending until the opt-in is completed.

The status shouldn't be "Added by admin" since they weren't – the user adds herself in this case.

Ideally, there would be an option in configuring the profile to set the "added to group" status, since I understand the use cases where we would want the status to be added by admin.