fix(polyfills): manually import reflect-metadata

fixes #2770

Author: Awk34

templates/app/client/app/polyfills.js

@@ -1,6 +1,7 @@
 // Enable certain polyfills depending on which browsers you need to support
 // import 'core-js/es6';
 // import 'core-js/es7/reflect';
+import 'reflect-metadata';
 
 import 'zone.js/dist/zone';