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:

https://code.briarproject.org/briar/briar/-/blob/beta-1.2.14/briar-android/src/main/java/org/briarproject/briar/android/contact/add/remote/NicknameFragment.java#L139

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 (closed).

Follow up to !28 (merged).