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/static_javascript.go

@@ -5,12 +5,14 @@
 package ui // import "miniflux.app/ui"
 
 import (
+	"fmt"
 	"net/http"
 	"time"
 
 	"miniflux.app/http/request"
 	"miniflux.app/http/response"
 	"miniflux.app/http/response/html"
+	"miniflux.app/http/route"
 	"miniflux.app/ui/static"
 )
 
@@ -23,8 +25,15 @@ func (h *handler) showJavascript(w http.ResponseWriter, r *http.Request) {
 	}
 
 	response.New(w, r).WithCaching(etag, 48*time.Hour, func(b *response.Builder) {
+		contents := static.JavascriptBundles[filename]
+
+		if filename == "service-worker" {
+			variables := fmt.Sprintf(`const OFFLINE_URL="%s";`, route.Path(h.router, "offline"))
+			contents = append([]byte(variables)[:], contents[:]...)
+		}
+
 		b.WithHeader("Content-Type", "text/javascript; charset=utf-8")
-		b.WithBody(static.JavascriptBundles[filename])
+		b.WithBody(contents)
 		b.Write()
 	})
 }