Add $plain property to Restangular results

frontend/app/components/api/api-v3/api-v3.config.ts

@@ -28,6 +28,8 @@
 
 function apiV3Config(apiV3, halTransform) {
   apiV3.addResponseInterceptor((data, operation, what) => {
+    data.$plain = angular.copy(data);
+
     apiV3.addElementTransformer(what, halTransform);
 
     if (data && data._type === 'Collection') {

frontend/app/components/api/api-v3/hal/hal-resource.service.ts

@@ -47,7 +47,7 @@ function halResource(halTransform, HalLink, $q) {
     }
 
     constructor(protected $source, public $loaded = true) {
-      var source = $source.restangularized ? $source.plain() : angular.copy($source);
+      var source = angular.copy($source.restangularized ? $source.$plain : $source);
 
       this.$links = this.transformLinks();
       this.$embedded = this.transformEmbedded();