Exposed announcement settings in Content API

refs https://github.com/TryGhost/Team/issues/3011

- Allows to read following settings fields:
  - announcement - alias to announcement_content
  - announcement_background

ghost/core/core/shared/settings-cache/public.js

@@ -39,5 +39,7 @@ module.exports = {
     portal_plans: 'portal_plans',
     portal_name: 'portal_name',
     portal_button: 'portal_button',
-    comments_enabled: 'comments_enabled'
+    comments_enabled: 'comments_enabled',
+    announcement: 'announcement_content',
+    announcement_background: 'announcement_background'
 };

ghost/core/test/e2e-api/content/__snapshots__/settings.test.js.snap

@@ -5,6 +5,8 @@ Object {
   "meta": Object {},
   "settings": Object {
     "accent_color": "#FF1A75",
+    "announcement": null,
+    "announcement_background": "dark",
     "codeinjection_foot": null,
     "codeinjection_head": null,
     "comments_enabled": "off",

ghost/core/test/e2e-api/shared/__snapshots__/version.test.js.snap

@@ -1339,6 +1339,8 @@ Object {
   "meta": Object {},
   "settings": Object {
     "accent_color": "#FF1A75",
+    "announcement": null,
+    "announcement_background": "dark",
     "codeinjection_foot": null,
     "codeinjection_head": null,
     "comments_enabled": "off",
@@ -1435,6 +1437,8 @@ Object {
   "meta": Object {},
   "settings": Object {
     "accent_color": "#FF1A75",
+    "announcement": null,
+    "announcement_background": "dark",
     "codeinjection_foot": null,
     "codeinjection_head": null,
     "comments_enabled": "off",