Add 'beforeCreate' hook in rest model

app/assets/javascripts/discourse/models/rest.js.es6

@@ -3,6 +3,7 @@ const RestModel = Ember.Object.extend({
   isCreated: Ember.computed.equal('__state', 'created'),
   isSaving: false,
 
+  beforeCreate() { },
   afterUpdate() { },
 
   update(props) {
@@ -34,6 +35,8 @@ const RestModel = Ember.Object.extend({
 
     props = props || this.createProperties();
 
+    this.beforeCreate(props);
+
     const type = this.get('__type'),
           store = this.get('store'),
           adapter = store.adapterFor(type);