Add basic PWA offline page

- Remove feed_icons cache because it's causing more problems that it solve.
- Add basic offline mode when using the service worker.
- Starting in Chrome 93, offline mode is going to be a requirement to install the PWA.

https://developer.chrome.com/blog/improved-pwa-offline-detection/#enforcement-starting-chrome-93-august-2021

ui/middleware.go

@@ -142,7 +142,8 @@ func (m *middleware) isPublicRoute(r *http.Request) bool {
 		"webManifest",
 		"robots",
 		"sharedEntry",
-		"healthcheck":
+		"healthcheck",
+		"offline":
 		return true
 	default:
 		return false