Don't call render() after a POST, always use a redirect().

+ After a failed login, instead of just rendering the "login" view on the POST URL
  for DoLogin, we need to redirect to the Login URL, passing through the redirect
  parameter.
+ After a successful login, instead of just rendering the "welcome" view on the
  POST URL for DoLogin, we need to redirect to either the passed-in redirect URL,
  or to the home page / dashboard URL.