Add Service Work(e|a)r(round)

uses the promise in returned by .bootstrapModule
to register the serviceworker if required.
Seems to work reliable, even in unstable environments.
Requires production mode to comprehend.
(at least ng build --prod)

client/src/main.ts

@@ -10,4 +10,9 @@ if (environment.production) {
 
 platformBrowserDynamic()
     .bootstrapModule(AppModule)
+    .then(() => {
+        if ('serviceWorker' in navigator && environment.production) {
+            navigator.serviceWorker.register('ngsw-worker.js');
+        }
+    })
     .catch(err => console.log(err));