[Conversations Page] Replace `messagesByConversation` with `Conversation.messages`

[crispyricepc]

Instead of https://github.com/dsmessenger/mvp-react/blob/3746f4b06acc92ae8c6d17b19c7a5e2b92acdb27/src/components/ChatMessageView.jsx#L9-L31 we can use:

query MessagesByConversation( 
 $conversationId: Int! 
 $nMessages: Int! 
 $token: String! 
) { 
  conversation(id: $conversationId, token: $token) {
    messages(nMessages: $nMessages) {
      sender { username }
      timestamp
      revision
      content
    }
  }
}