code: function topLevelRedirect (url) {
    if (window === top) location.href = url
    else parent.postMessage('slackin-redirect:' + id + ':' + url, '*')
    // Q: Why can't we just `top.location.href = url;`?
    // A:
    // [sandboxing]: http://www.html5rocks.com/en/tutorials/security/sandboxed-iframes/
    // [CSP]: http://www.html5rocks.com/en/tutorials/security/content-security-policy/
    // [nope]: http://output.jsbin.com/popawuk/16
  }
docs: redirect, using "RPC" to parent if necessary