Add support for email listserve for ppa users

At the moment there is no way for a ppa developer to communicate with the users of the ppa, apart from those that wants to become members of the developing team. At times, there are bugs or issues in the ppa that need to be communicated to the users, but currently there is no way to do that. I think it would be very beneficial for a user to add the option to add his/her email address at the time of the subscription to a ppa.

One possible implementation could be the following:

1. A user finds a PPA of interest and s/he subscribes to it as indicated from the ppa page.
2. Usually the process stops here. Instead, a panel should come up asking if they would be interested to be contacted regarding issues/bugs/communication regarding the PPA.
3. The ppa-developer would not necessarily have access to the list (for privacy reason), but only to a listserve address that s/he could use to communicate with the users.