Refactored posts browsing to posts service

refs https://github.com/TryGhost/Team/issues/3423

- This refactor allow for smaller and cleaner change that's coming up when dealing with `?collection=` query parameter

ghost/core/core/server/api/endpoints/posts.js

@@ -69,7 +69,7 @@ module.exports = {
             unsafeAttrs: unsafeAttrs
         },
         query(frame) {
-            return models.Post.findPage(frame.options);
+            return postsService.browsePosts(frame.options);
         }
     },
 

ghost/posts-service/lib/PostsService.js

@@ -27,6 +27,15 @@ class PostsService {
         this.collectionsService = collectionsService;
     }
 
+    /**
+     *
+     * @param {Object} options - frame options
+     * @returns {Promise<Object>}
+     */
+    async browsePosts(options) {
+        return this.models.Post.findPage(options);
+    }
+
     async readPost(frame) {
         const model = await this.models.Post.findOne(frame.data, frame.options);