Favor createdAt over timestamp

- it's more human readable and consistent with all other nodes in
  production

backend/src/models/Migration.js

@@ -1,5 +1,5 @@
 export default {
   title: { type: 'string', primary: true, token: true },
   description: { type: 'string' },
-  timestamp: { type: 'number', unique: true },
+  createdAt: { type: 'string', isoDate: true, default: () => new Date().toISOString() },
 }