Added resource caching to Posts/Pages Content API

refs https://github.com/TryGhost/Toolbox/issues/522

- Browse endpoints for Posts and Pages are creating the most database traffic in the system. These are read-only endpoints that don't have to be fresh 100% of the time. Having optional cache allows to offload some of the database querying to more efficient storage.
- To enable cache for Posts/Pages browse endpoints there are two prerequisites:
  - set 'hostSettings:postsPublicCache:enabled' to 'true' in the configuration file
  - add 'postsPublic' cache adapter in cache configuration

- Example config for adapters with 60s TTL for a cached resource:
```
    "adapters": {
        "cache": {
            "postsPublic": {
                "adapter": "Redis",
                "ttl": 60,
                "keyPrefix": "site_id_here:posts-content-api:"
            }
        }
    },
```

ghost/core/core/boot.js

@@ -295,6 +295,7 @@ async function initServices({config}) {
     const emailAnalytics = require('./server/services/email-analytics');
     const mentionsService = require('./server/services/mentions');
     const tagsPublic = require('./server/services/tags-public');
+    const postsPublic = require('./server/services/posts-public');
 
     const urlUtils = require('./shared/url-utils');
 
@@ -313,6 +314,7 @@ async function initServices({config}) {
         members.init(),
         tiers.init(),
         tagsPublic.init(),
+        postsPublic.init(),
         membersEvents.init(),
         permissions.init(),
         xmlrpc.listen(),

ghost/core/core/server/api/endpoints/pages-public.js

@@ -1,6 +1,8 @@
 const tpl = require('@tryghost/tpl');
 const errors = require('@tryghost/errors');
 const models = require('../../models');
+const postsPublicService = require('../../services/posts-public');
+
 const ALLOWED_INCLUDES = ['tags', 'authors', 'tiers'];
 
 const messages = {
@@ -34,7 +36,7 @@ module.exports = {
         },
         permissions: true,
         query(frame) {
-            return models.Post.findPage(frame.options);
+            return postsPublicService.api.browse(frame.options);
         }
     },
 

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

@@ -1,6 +1,8 @@
 const models = require('../../models');
 const tpl = require('@tryghost/tpl');
 const errors = require('@tryghost/errors');
+const postsPublicService = require('../../services/posts-public');
+
 const allowedIncludes = ['tags', 'authors', 'tiers', 'sentiment'];
 
 const messages = {
@@ -34,7 +36,7 @@ module.exports = {
         },
         permissions: true,
         query(frame) {
-            return models.Post.findPage(frame.options);
+            return postsPublicService.api.browse(frame.options);
         }
     },
 

ghost/core/core/server/services/posts-public/index.js

@@ -0,0 +1 @@
+module.exports = require('./service');

ghost/core/core/server/services/posts-public/service.js

@@ -0,0 +1,31 @@
+class PostsPublicServiceWrapper {
+    async init() {
+        if (this.api) {
+            // Already done
+            return;
+        }
+
+        // Wire up all the dependencies
+        const {Post} = require('../../models');
+        const adapterManager = require('../adapter-manager');
+        const config = require('../../../shared/config');
+
+        let postsCache;
+        if (config.get('hostSettings:postsPublicCache:enabled')) {
+            postsCache = adapterManager.getAdapter('cache:postsPublic');
+        }
+
+        const {PublicResourcesRepository} = require('@tryghost/public-resource-repository');
+
+        this.postsRepository = new PublicResourcesRepository({
+            Model: Post,
+            cache: postsCache
+        });
+
+        this.api = {
+            browse: this.postsRepository.getAll.bind(this.postsRepository)
+        };
+    }
+}
+
+module.exports = new PostsPublicServiceWrapper();