Add service worker to cache feed icons

ui/static/js/bootstrap.js

@@ -71,4 +71,11 @@ document.addEventListener("DOMContentLoaded", function() {
         mouseHandler.onClick(".logo", () => menuHandler.toggleMainMenu());
         mouseHandler.onClick(".header nav li", (event) => menuHandler.clickMenuListItem(event));
     }
+
+    if ("serviceWorker" in navigator) {
+        let scriptElement = document.getElementById("service-worker-script");
+        if (scriptElement) {
+            navigator.serviceWorker.register(scriptElement.src);
+        }
+    }
 });

ui/static/js/sw.js

@@ -0,0 +1,14 @@
+self.addEventListener("fetch", (event) => {
+    if (event.request.url.includes("/feed/icon/")) {
+        event.respondWith(
+            caches.open("feed_icons").then((cache) => {
+                return cache.match(event.request).then((response) => {
+                    return response || fetch(event.request).then((response) => {
+                        cache.put(event.request, response.clone());
+                        return response;
+                    });
+                });
+            })
+        );
+    }
+});