Fixed page actions stored under page resource type

refs bde9b84221

- the commit above claimed to fix an issue where actions taken on pages
  were stored using the `post` resource type
- whilst it does that, this actually breaks part of the API because we
  use the `resource_type` to join on tables when doing an
  `include=resource`
- this commit reverts that functionality and instead stores the type in
  the `context` field so we can still differentiate between pages and
  posts

ghost/core/core/server/models/post.js

@@ -39,9 +39,7 @@ Post = ghostBookshelf.Model.extend({
     tableName: 'posts',
 
     actionsCollectCRUD: true,
-    actionsResourceType: function () {
-        return this.get('type') || this.previous('type');
-    },
+    actionsExtraContext: ['type'],
 
     /**
      * @NOTE