Define belongsToMany foreign keys for tags in the model layer

no issue

- otherwise we will have trouble in the future fetching relations by foreign key
  - e.g. `tag_id: {id}`
  - this won't work if we don't explicitly define the name of the keys
  - bookshelf can't fulfil the request
- this does not change any behaviour, it just makes use of the ability to define the names of your foreign keys

core/server/models/post.js

@@ -331,7 +331,9 @@ Post = ghostBookshelf.Model.extend({
     },
 
     tags: function tags() {
-        return this.belongsToMany('Tag').withPivot('sort_order').query('orderBy', 'sort_order', 'ASC');
+        return this.belongsToMany('Tag', 'posts_tags', 'post_id', 'tag_id')
+            .withPivot('sort_order')
+            .query('orderBy', 'sort_order', 'ASC');
     },
 
     fields: function fields() {