Merge branch 'use-contact-id-as-conversation-id' into 'master'

Use contact ID rather than messaging group ID to identify conversation

We originally used the private messaging group ID to identify the private conversation, but now that the conversation includes messages from multiple clients it's more appropriate to use the contact ID.

This refactoring isn't urgent - I've had the branch lying around for a while, but I'm putting it up for review because #734 will touch some of the same code.

See merge request !386