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/offline.go

@@ -0,0 +1,20 @@
+// Copyright 2021 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package ui // import "miniflux.app/ui"
+
+import (
+	"net/http"
+
+	"miniflux.app/http/request"
+	"miniflux.app/http/response/html"
+	"miniflux.app/ui/session"
+	"miniflux.app/ui/view"
+)
+
+func (h *handler) showOfflinePage(w http.ResponseWriter, r *http.Request) {
+	sess := session.New(h.store, request.SessionID(r))
+	view := view.New(h.tpl, r, sess)
+	html.OK(w, r, view.Render("offline"))
+}