Exposed site SEO data through Conent API & {{@site.*}} helper (#10925)

refs #10921

- Site SEO data will now be available as part of `GET /settings` response in Content API as well as part of {{@site.*}} helper

core/server/services/settings/public.js

@@ -18,5 +18,13 @@ module.exports = {
     active_timezone: 'timezone',
     ghost_head: 'ghost_head',
     ghost_foot: 'ghost_foot',
-    navigation: 'navigation'
+    navigation: 'navigation',
+    meta_title: 'meta_title',
+    meta_description: 'meta_description',
+    og_image: 'og_image',
+    og_title: 'og_title',
+    og_description: 'og_description',
+    twitter_image: 'twitter_image',
+    twitter_title: 'twitter_title',
+    twitter_description: 'twitter_description'
 };

core/test/acceptance/old/content/settings_spec.js

@@ -43,6 +43,7 @@ describe('Settings Content API', function () {
 
                 // Verify we have the right keys for settings
                 settings.should.have.properties(_.values(publicSettings));
+                Object.keys(settings).length.should.equal(22);
 
                 // Verify that we are returning the defaults for each value
                 _.forEach(settings, (value, key) => {