Fix local_part uniqueness of mailman
We validated uniqueness of local_part only if the EmailIaddress to save is a EmailMailman. BUT we did not ensure that we check only against other mailman lists.
If there is a Schleuder and Mailman list with the same local_part & domain, this will still be catched by then general uniqueness
Background: I tried to delete a mailman list with the same local_part as an already existing (deleted) address.