Add service worker and offline page (needed to install the webapp on mobile devices)

public/serviceworker.js

@@ -0,0 +1,58 @@
+/*
+ * serviceworker.js
+ * Copyright (c) 2021 james@firefly-iii.org
+ *
+ * This file is part of Firefly III (https://github.com/firefly-iii).
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+let staticCachePrefix = "firefly-III-"
+let staticCacheName = staticCachePrefix + new Date().getTime();
+let cachedFiles = [
+    '/offline',
+    '/v2/plugins/local-fonts/gf-source.css',
+    '/v2/css/app.css',
+];
+
+// Create cache on install
+self.addEventListener("install", event => {
+    this.skipWaiting();
+    event.waitUntil(
+        caches.open(staticCacheName).then(cache => cache.addAll(cachedFiles))
+    )
+});
+
+// Clear cache on activate
+self.addEventListener('activate', event => {
+    event.waitUntil(
+        caches.keys().then(cacheNames => {
+            return Promise.all(
+                cacheNames
+                    .filter(cacheName => (cacheName.startsWith(staticCachePrefix)))
+                    .filter(cacheName => (cacheName !== staticCacheName))
+                    .map(cacheName => caches.delete(cacheName))
+            );
+        })
+    );
+});
+
+// Serve from Cache or return the offline page
+self.addEventListener("fetch", event => {
+    event.respondWith(
+        caches.match(event.request)
+            .then(response => (response || fetch(event.request)))
+            .catch(() => caches.match('offline'))
+    )
+});