allow the creation of forward to mailbox
Otherwise, even if one wanted to have two forwards to one mailbox, you cannot create them, because they are created one-by-one.
As discussed in !44 (closed) it is maybe better to just allow this and think about a better UX in the admin panel.
E.g. we could also just have a combined "create alias/fwd" form in at the frontend, that does the "right thing" towards iapi.