Added bookshelf model for collections

We setup the relations and NQL expansions/replacements here rather than in the
repository, we want to keep all the bookshelf code together in one place.

ghost/core/core/server/models/collection.js

@@ -0,0 +1,78 @@
+const ghostBookshelf = require('./base');
+const urlUtils = require('../../shared/url-utils');
+
+const Collection = ghostBookshelf.Model.extend({
+    tableName: 'collections',
+
+    formatOnWrite(attrs) {
+        if (attrs.feature_image) {
+            attrs.feature_image = urlUtils.toTransformReady(attrs.feature_image);
+        }
+        return attrs;
+    },
+
+    parse() {
+        const attrs = ghostBookshelf.Model.prototype.parse.apply(this, arguments);
+
+        if (attrs.feature_image) {
+            attrs.feature_image = urlUtils.transformReadyToAbsolute(attrs.feature_image);
+        }
+
+        return attrs;
+    },
+
+    relationships: ['posts'],
+    relationshipConfig: {
+        posts: {
+            editable: false
+        }
+    },
+
+    relationshipBelongsTo: {
+        posts: 'posts'
+    },
+
+    filterExpansions() {
+        return [{
+            key: 'posts',
+            replacement: 'posts.id'
+        }];
+    },
+
+    filterRelations() {
+        return {
+            posts: {
+                tableName: 'posts',
+                type: 'manyToMany',
+                joinTable: 'collections_posts',
+                joinFrom: 'collection_id',
+                joinTo: 'post_id'
+            }
+        };
+    },
+
+    permittedAttributes() {
+        let filteredKeys = ghostBookshelf.Model.prototype.permittedAttributes.apply(this, arguments);
+
+        this.relationships.forEach((key) => {
+            filteredKeys.push(key);
+        });
+
+        return filteredKeys;
+    },
+
+    posts() {
+        return this.belongsToMany(
+            'Post',
+            'collections_posts',
+            'collection_id',
+            'post_id',
+            'id',
+            'id'
+        );
+    }
+});
+
+module.exports = {
+    Collection: ghostBookshelf.model('Collection', Collection)
+};