Two-way invitation for same chat

This might cause some problems. Nowadays, opening a contact without an open chat creates one; doing so with an ongoing chat opens it. However, if both create a chat with their interlocutor without accepting invitations, we will have duplicate invitations, and possibly, duplicate chats.

The issue names no files, tests, or entry points. Start by tracing how opening a contact creates or opens a chat and how invitations are accepted; done means two people cannot create duplicate invitations or duplicate chats for the same conversation.

Written by the indexing model from the issue text.