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.