- Introduces qualified users in NewConv
- Checks if remote users exist upon creating a conversation
- Adds remote users to the database
- Adds integration tests for qualified users
- Updates golden tests related to NewConv
- Add fromJSON test for NewConv to establish backwards-compatibility.
  This is to make sure the `qualified_users` is indeed optional
  in the JSON being parsed.

Co-authored-by: Paolo Capriotti <paolo@capriotti.io>