Wrap content type attributes in its object

and lookup the 'attributes' object during linking

Author: scriptype

src/ssg/e2e-tests/magazine/fixtures/fs/schema/Person.md

@@ -1,8 +1,9 @@
 ---
 name: Person
-articles: [+Article:author]
-demos: [+Demo:maker]
-events: [+Event:organizers, +Event:participants]
-books: [+Book:author]
-twin: +Person:twin
+attributes:
+  articles: [+Article:author]
+  demos: [+Demo:maker]
+  events: [+Event:organizers, +Event:participants]
+  books: [+Book:author]
+  twin: +Person:twin
 ---

src/ssg/lib/linking.js

@@ -31,8 +31,8 @@ function findLinkedNode(nodes, linkPath) {
 
 function addLinkBack(sourceNode, targetNode, key) {
   if (sourceNode.schema) {
-    Object.keys(sourceNode.schema).forEach(schemaKey => {
-      const schemaValue = sourceNode.schema[schemaKey]
+    Object.keys(sourceNode.schema.attributes || []).forEach(schemaKey => {
+      const schemaValue = sourceNode.schema.attributes[schemaKey]
       const isSchemaValueArray = Array.isArray(schemaValue)
       const re = new RegExp(`^\\+(${targetNode.contentType}|):${key}$`)
       const match = isSchemaValueArray ?