All conversation messages are marked as read via messaging manager
The setReadFlag() method is declared in the ConversationClient interface, but only MessagingManagerImpl's implementation of the method is used (except for one integration test).
If clients don't need their own implementations, the method could be moved to the MessagingManager or ConversationManager interface, and the unused implementation in ConversationClientImpl could be removed.
On the other hand, if clients do need their own implementations then the implementation in ConversationClientImpl should be updated and we should make sure that ConversationViewModel and (eventually) MessagingControllerImpl call the right client for each message.
- repeat tests from !1389 (merged)