Updated responses to mass subscribe/invite to be more precise.

Fixes #481 (closed) This changes the The address %(address)s has been subscribed to %(list)s. to The address %(address)s has been invited to %(list)s. for invitations and appends Subscription may be pending address verification, confirmation or approval as appropriate. to the subscribe response if not all the pre_approved, pre_confirmed and pre_verified boxes are checked.