Update cache control for web assets

The commit [1] introduced the `cache-control` header field with the value
`no-cache`. This enforces any cache to revalidate. Apparently this does
not prohibit *FireFox* from caching the assets. `no-cache, no-store,
must-revalidate` does by prohibiting to even save anything of the
request.

[1] c3850708c1

ui/web.go

@@ -40,7 +40,7 @@ func serveAsset(w http.ResponseWriter, req *http.Request, fp string) {
 		return
 	}
 
-	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Cache-Control", "no-cache, no-store, must-revalidate")
 	http.ServeContent(w, req, info.Name(), info.ModTime(), bytes.NewReader(file))
 }