Added pages.hide_title_and_feature_image to admin-api-schema

closes TryGhost/Product#3556

- adds new property to pages schema to allow passthrough of the value when adding/updating via API

Author: kevinansfield

packages/admin-api-schema/lib/schemas/pages.json

@@ -126,6 +126,9 @@
           "format": "uri-reference",
           "maxLength": 2000
         },
+        "hide_title_and_feature_image": {
+          "type": "boolean"
+        },
         "authors": {
           "$ref": "#/definitions/page-authors"
         },