Move to centralized message/cache data layer

Also, ensure that conversation.messageCollection has nothing in it
unless it has an associated ConversationView.

js/modules/types/message.js

@@ -498,6 +498,8 @@ exports.createAttachmentDataWriter = ({
       });
     };
 
+    // TODO: need to handle attachment thumbnails and video screenshots
+
     const messageWithoutAttachmentData = Object.assign(
       {},
       await writeThumbnails(message, { logger }),