Include last private chat activity in list of contacts

In order to sort the contact list in Briar GTK chronologically (briar-gtk#4 (closed)), I thought of adding a lastChatActivity key to the list of contacts exposed by Briar Headless. The value of this key would be the timestamp of the last message in the private chat with that contact.

I deliberately called it lastChatActivity to leave lastActivity available for other purposed, i.e. the contact's last message or the last time we were connected to them.