Move to centralized message/cache data layer

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

ts/backbone/Conversation.ts

@@ -1,76 +0,0 @@
-import is from '@sindresorhus/is';
-
-import { Collection as BackboneCollection } from '../types/backbone/Collection';
-import { deferredToPromise } from '../../js/modules/deferred_to_promise';
-import { IndexableBoolean } from '../types/IndexedDB';
-import { Message } from '../types/Message';
-
-export const fetchVisualMediaAttachments = async ({
-  conversationId,
-  count,
-  WhisperMessageCollection,
-}: {
-  conversationId: string;
-  count: number;
-  WhisperMessageCollection: BackboneCollection<Message>;
-}): Promise<Array<Message>> =>
-  fetchFromAttachmentsIndex({
-    name: 'hasVisualMediaAttachments',
-    conversationId,
-    WhisperMessageCollection,
-    count,
-  });
-
-export const fetchFileAttachments = async ({
-  conversationId,
-  count,
-  WhisperMessageCollection,
-}: {
-  conversationId: string;
-  count: number;
-  WhisperMessageCollection: BackboneCollection<Message>;
-}): Promise<Array<Message>> =>
-  fetchFromAttachmentsIndex({
-    name: 'hasFileAttachments',
-    conversationId,
-    WhisperMessageCollection,
-    count,
-  });
-
-const fetchFromAttachmentsIndex = async ({
-  name,
-  conversationId,
-  WhisperMessageCollection,
-  count,
-}: {
-  name: 'hasVisualMediaAttachments' | 'hasFileAttachments';
-  conversationId: string;
-  WhisperMessageCollection: BackboneCollection<Message>;
-  count: number;
-}): Promise<Array<Message>> => {
-  if (!is.string(conversationId)) {
-    throw new TypeError("'conversationId' is required");
-  }
-
-  if (!is.object(WhisperMessageCollection)) {
-    throw new TypeError("'WhisperMessageCollection' is required");
-  }
-
-  const collection = new WhisperMessageCollection();
-  const lowerReceivedAt = 0;
-  const upperReceivedAt = Number.MAX_VALUE;
-  const condition: IndexableBoolean = 1;
-  await deferredToPromise(
-    collection.fetch({
-      index: {
-        name,
-        lower: [conversationId, lowerReceivedAt, condition],
-        upper: [conversationId, upperReceivedAt, condition],
-        order: 'desc',
-      },
-      limit: count,
-    })
-  );
-
-  return collection.models.map(model => model.toJSON());
-};

ts/backbone/index.ts

@@ -1,4 +1,3 @@
-import * as Conversation from './Conversation';
 import * as Views from './views';
 
-export { Conversation, Views };
+export { Views };