Moved setId function to Bookshelf events plugin

no issue

- `setId` is only used within the `events` plugin and it makes sense to
  keep code together
- we don't lose anything by putting it here, but it should make it
  easier to test in the future

core/server/models/base/index.js

@@ -7,7 +7,6 @@
 
 // All other parts of Ghost, including the frontend & admin UI are only allowed to access data via the API.
 const moment = require('moment');
-const ObjectId = require('bson-objectid');
 const schema = require('../../data/schema');
 
 const ghostBookshelf = require('./bookshelf');
@@ -51,14 +50,6 @@ ghostBookshelf.Model = ghostBookshelf.Model.extend({
         return moment(this.get(attr)).diff(moment(this.previous(attr))) !== 0;
     },
 
-    /**
-     * we auto generate a GUID for each resource
-     * no auto increment
-     */
-    setId: function setId() {
-        this.set('id', ObjectId().toHexString());
-    },
-
     wasChanged() {
         /**
          * @NOTE:

core/server/models/base/plugins/events.js

@@ -1,5 +1,6 @@
 const _ = require('lodash');
 const debug = require('@tryghost/debug')('models:base:model-events');
+const ObjectId = require('bson-objectid');
 
 const events = require('../../../lib/common/events');
 const schema = require('../../../data/schema');
@@ -31,6 +32,14 @@ module.exports = function (Bookshelf) {
             this.on('saved', this.onSaved);
         },
 
+        /**
+         * we auto generate a GUID for each resource
+         * no auto increment
+         */
+        setId: function setId() {
+            this.set('id', ObjectId().toHexString());
+        },
+
         /**
          * @NOTE
          * We have to remember the `_previousAttributes` attributes, because when destroying resources