Always serve plugins folder with or without the --plugins

without --plugins, serve at `plugins_`, otherwise serve at `plugins`

Change-Id: I91202656ec60501fc826550294e5c5a48479ccca

polygerrit-ui/server.go

@@ -86,6 +86,9 @@ func main() {
 		log.Println("Local plugins from", "../plugins")
 	} else {
 		http.HandleFunc("/plugins/", handleProxy)
+		// Serve local plugins from `plugins_`
+		http.Handle("/plugins_/", http.StripPrefix("/plugins_/",
+			http.FileServer(http.Dir("../plugins"))))
 	}
 	log.Println("Serving on port", *port)
 	log.Fatal(http.ListenAndServe(*port, &server{}))