Added an internal API for email-only posts

refs https://github.com/TryGhost/Team/issues/899

- The internal API is needed to be able to fetch email-only posts through email router. The concept is similar to Preview API with a difference that only posts with `sent` status are accessible and there is content-gating present.

core/server/api/canary/email-post.js

@@ -0,0 +1,41 @@
+const i18n = require('../../../shared/i18n');
+const errors = require('@tryghost/errors');
+const models = require('../../models');
+const ALLOWED_INCLUDES = ['authors', 'tags'];
+
+module.exports = {
+    docName: 'email_post',
+
+    read: {
+        permissions: true,
+        options: [
+            'include'
+        ],
+        data: [
+            'slug'
+        ],
+        validation: {
+            options: {
+                include: {
+                    values: ALLOWED_INCLUDES
+                }
+            },
+            data: {
+                slug: {
+                    required: true
+                }
+            }
+        },
+        async query(frame) {
+            const model = await models.Post.findOne(Object.assign(frame.data, {status: 'sent'}), frame.options);
+
+            if (!model) {
+                throw new errors.NotFoundError({
+                    message: i18n.t('errors.api.posts.postNotFound')
+                });
+            }
+
+            return model;
+        }
+    }
+};

core/server/api/canary/index.js

@@ -107,6 +107,10 @@ module.exports = {
         return shared.pipeline(require('./preview'), localUtils);
     },
 
+    get emailPost() {
+        return shared.pipeline(require('./email-post'), localUtils);
+    },
+
     get oembed() {
         return shared.pipeline(require('./oembed'), localUtils);
     },

core/server/api/canary/utils/serializers/output/email-posts.js

@@ -0,0 +1,13 @@
+const mapper = require('./utils/mapper');
+const gating = require('./utils/post-gating');
+
+module.exports = {
+    read(model, apiConfig, frame) {
+        const emailPost = mapper.mapPost(model, frame);
+        gating.forPost(emailPost, frame);
+
+        frame.response = {
+            email_posts: [emailPost]
+        };
+    }
+};

core/server/api/canary/utils/serializers/output/index.js

@@ -91,6 +91,10 @@ module.exports = {
         return require('./preview');
     },
 
+    get email_post() {
+        return require('./email-posts');
+    },
+
     get oembed() {
         return require('./oembed');
     },