Display avatars according to the server config

This uses a similar pattern to
https://gerrit-review.googlesource.com/72659/ where a promise is
used for a global var that is retrieved asynchronously.

Change-Id: I99a4b589d368876becf4dd000a2ffde39b34eaa4

polygerrit-ui/server.go

@@ -47,6 +47,7 @@ func main() {
 
 	http.HandleFunc("/changes/", handleRESTProxy)
 	http.HandleFunc("/accounts/", handleRESTProxy)
+	http.HandleFunc("/config/", handleRESTProxy)
 	log.Println("Serving on port", *port)
 	log.Fatal(http.ListenAndServe(*port, &server{}))
 }