chore(pwa): bump cache to v2, include variants.js, evict old caches

sw.js

@@ -1,10 +1,11 @@
-const CACHE_NAME = 'dienstplan-pro-v1';
+const CACHE_NAME = 'dienstplan-pro-v2';
 const ASSETS = [
   './',
   './index.html',
   './styles.css',
   './app.js',
   './calculator.js',
+  './variants.js',
   './holidays.js',
   './storage.js'
 ];
@@ -15,6 +16,14 @@ self.addEventListener('install', (e) => {
   );
 });
 
+self.addEventListener('activate', (e) => {
+  e.waitUntil(
+    caches.keys().then(keys => Promise.all(
+      keys.filter(k => k !== CACHE_NAME).map(k => caches.delete(k))
+    ))
+  );
+});
+
 self.addEventListener('fetch', (e) => {
   e.respondWith(
     caches.match(e.request).then((response) => response || fetch(e.request))