Drupal - How to redirect to a page after submitting a form

In the submit handler you do

Drupal 7:

$form_state['redirect'] = 'url';

Drupal 8:

$form_state->setRedirect('route', $args, $options);
$form_state->setRedirectUrl(\Drupal\Core\Url);

It should be noted there are several triggers that may prevent a redirection:

• $form_state['redirect'] === FALSE: If set, the form builder function or form validation/submit handler does not want a user to be redirected, which means that drupal_goto() is not invoked. For most forms, the redirection logic will be the same regardless of whether $form_state['redirect'] is undefined or FALSE. However, in case it was not defined and the current request contains a 'destination' query string, drupal_goto() will redirect to that given destination instead. Only setting $form_state['redirect'] to FALSE will prevent any redirection.
• $form_state['no_redirect'] === TRUE: When set, the callback that originally built the form explicitly disallows any redirection, regardless of the redirection value in $form_state['redirect']. For example, ajax_get_form() defines $form_state['no_redirect'] when building a form in an AJAX callback to prevent any redirection. $form_state['no_redirect'] should NOT be altered by form builder functions or form validation/submit handlers.
• $form_state['programmed'] === TRUE: means the form submission was usually invoked via drupal_form_submit(), so any redirection would break the script that invoked drupal_form_submit().
• $form_state['rebuild'] === TRUE: means the form needs to be rebuilt without redirection.

For Drupal 7, I tried this on a custom module and it works. It can also be used in a theme:

function my_theme_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'contact_site_form') {
    $form['#submit'][] = 'contact_form_submit_handler';
  }
}

function contact_form_submit_handler(&$form, &$form_state) {
  $form_state['redirect'] = 'thank-you-page-alias';
}

BTW, I found this snippet here: https://gist.github.com/postrational/5768796