Add a warning for global function use like t() in classes

Sister issue of #2608738: Add a warning for \Drupal:: use in OO code.

t() should not be used in a controller class or a form class or a plugin class. Dependency injection should be used instead.

Wrong:

class TestForm extends FormBase {

  public function buildForm($form, $form_state) {
    $form['something'] = t('Example text');
  }

}

Correct:

class TestForm extends FormBase {

  public function buildForm($form, $form_state) {
    $form['something'] = $this->t('Example text');
  }

}