Verify unverified contacts
Contacts who are added via introductions (or via long-distance transports, if we decide to support that) are considered "unverified" due to the possibility of a man-in-the-middle attack against the key exchange protocol. Contacts who are added face-to-face are considered "verified".
Design and implement a protocol and UI for verifying the identity of a previously unverified contact when the user meets the contact face-to-face.