Add "MyGists" link to GitHub Gist's header

MyGists.user.js

// ==UserScript==
// @name        MyGists
// @namespace   jp.tkrkt
// @description Add MyGists Button on Gist
// @include     https://gist.github.com/*
// @version     3
// @grant       none
// ==/UserScript==

var meta = document.querySelector('meta[name=user-login]');

if (meta){
  var user = meta.getAttribute('content');
  var container = document.querySelector('.header-nav[role=navigation]');
  
  var li = document.createElement('li');
  li.setAttribute('class', 'header-nav-item');
  var a = document.createElement('a');
  a.setAttribute('class', 'header-nav-link');
  a.setAttribute('href', '/' + user);
  a.textContent = 'My Gists';
  li.appendChild(a);
  
  container.insertBefore(li, container.firstChild);
}