Forced "latest" Collection Entity to have no posts

refs https://github.com/TryGhost/Arch/issues/95

We're going to be treating the "latest" Collection as a "virtual" Collection
where we will hydrate the posts from the posts repository. This will allow for
more performant queries, and less work to keep the "latest" Collection in sync

ghost/collections/src/Collection.ts

@@ -136,6 +136,9 @@ export class Collection {
      * @param index {number} - The index to insert the post at, use negative numbers to count from the end.
      */
     addPost(post: CollectionPost, index: number = -0) {
+        if (this.slug === 'latest') {
+            return false;
+        }
         if (this.type === 'automatic') {
             const matchesFilter = this.postMatchesFilter(post);
 
@@ -268,7 +271,7 @@ export class Collection {
             createdAt: Collection.validateDateField(data.created_at, 'created_at'),
             updatedAt: Collection.validateDateField(data.updated_at, 'updated_at'),
             deleted: data.deleted || false,
-            posts: data.posts || []
+            posts: data.slug !== 'latest' ? (data.posts || []) : []
         });
     }
 }

ghost/collections/test/Collection.test.ts

@@ -339,6 +339,27 @@ describe('Collection', function () {
         assert(collection.posts[collection.posts.length - 2] === '3');
     });
 
+    it('Does not add a post to the latest collection', async function () {
+        const collection = await Collection.create({
+            title: 'Testing adding to latest',
+            slug: 'latest',
+            type: 'automatic',
+            filter: ''
+        });
+
+        assert.equal(collection.posts.length, 0, 'Collection should have no posts');
+
+        const added = await collection.addPost({
+            id: '0',
+            featured: false,
+            published_at: new Date(),
+            tags: []
+        });
+
+        assert.equal(added, false);
+        assert.equal(collection.posts.length, 0, 'The non-featured post should not have been added');
+    });
+
     it('Adds a post to an automatic collection when it matches the filter', async function () {
         const collection = await Collection.create({
             title: 'Testing adding posts',