code: func login(w http.ResponseWriter, r *http.Request) {
  	r.ParseForm()
  
  	username := r.FormValue("username")
  	password := r.FormValue("password")
  	linkingToken := r.FormValue("account_linking_token")
  	rawRedirect := r.FormValue("redirect_uri")
  
  	if !checkCredentials(username, password) {
  		fmt.Fprint(w, templateLogin(loginPath, linkingToken, rawRedirect, true))
  		return
  	}
  
  	redirectURL, err := url.Parse(rawRedirect)
  	if err != nil {
  		log.Println("failed to parse url:", err)
  		return
  	}
  
  	q := redirectURL.Query()
  	q.Set("authorization_code", "something")
  	redirectURL.RawQuery = q.Encode()
  
  	w.Header().Set("Location", redirectURL.String())
  	w.WriteHeader(http.StatusFound)
  }
docs: // login is the endpoint that handles the actual signing in, by
  // checking the credentials, then redirecting to Facebook Messenger if
  // they are valid.