Warn users of potential attack when creating contacts
@grote or @akwizgran informed me of this potential attack back in the old days of Briar GTK. An attacker might try to find out which contacts a user has, which we try to prevent by implementing the following logic from the Android client:
Also see the respective MR at Briar GTK briar-gtk!97 (merged) where I tried to re-implement it, but we should follow the Android's logic because that one got peer reviewed.
Sub task of #81.
Follow up to !28 (merged).