Include last private chat activity in list of contacts

Contact items now look like this:

{
    "author": {
        "formatVersion": 1,
        "id": "y1wkIzAimAbYoCGgWxkWlr6vnq1F8t1QRA/UMPgI0E0=",
        "name": "Test",
        "publicKey": "BDu6h1S02bF4W6rgoZfZ6BMjTj/9S9hNN7EQoV05qUo="
    },
    "contactId": 1,
    "alias" : "A local nickname",
    "handshakePublicKey": "XnYRd7a7E4CTqgAvh4hCxh/YZ0EPscxknB9ZcEOpSzY=",
    "verified": true,
    "lastChatActivity": 1557838312175
}

Fixes #1722 (closed).