Fixed page actions events stored as posts

- this was due to the fact that we use the same model for pages as we do
  for posts, so the hardcoded `post` key was not accurate
- this commit adds support for providing a function to return the key for the
  action type

ghost/core/core/server/models/base/plugins/actions.js

@@ -23,11 +23,17 @@ module.exports = function (Bookshelf) {
                 return;
             }
 
+            let resourceType = this.actionsResourceType;
+
+            if (typeof resourceType === 'function') {
+                resourceType = resourceType.bind(this)();
+            }
+
             // @TODO: implement context
             return {
                 event: event,
                 resource_id: this.id || this.previous('id'),
-                resource_type: this.actionsResourceType,
+                resource_type: resourceType,
                 actor_id: actor.id,
                 actor_type: actor.type
             };

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

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