Expose the replacement of an already existing document by another instance as a conflict

When an instance start replication a document with an instance which already have one at the same location, the existing document is replaced without any kind of clue that this happens.

When a replication 3 ways merge conflict is identified, the involved authors get a notification and a banner is show to verify the content. We could also identity this kind of replacement as a conflict to have the same notifications and warning banner.