Inject translation service into feeds.pubsubhubbub service

Using t() isn't strictly wrong, this function is not deprecated, but it is encouraged to not use it when possible. The reason behind replacing t() with something else is to make it unit testable.

1. public static function validateUrl(&$element, FormStateInterface $form_state, &$complete_form) {
   -      $form_state->setError($element, t('The URI %url is not valid.', ['%url' => $value]));
   +      $form_state->setError($element, $this->t('The URI %url is not valid.', ['%url' => $value]));
   

   $this->t() cannot be used in static functions. Instead a new TranslatableMarkup object should be created.

2. --- a/src/EventSubscriber/PubSubHubbub.php
   +++ b/src/EventSubscriber/PubSubHubbub.php
   @@ -113,13 +113,13 @@ class PubSubHubbub implements EventSubscriberInterface {
        $subscription->subscribe();
    
        $batch = [
   -      'title' => t('Subscribing to: %title', ['%title' => $feed->label()]),
   -      'init_message' => t('Subscribing to: %title', ['%title' => $feed->label()]),
   +      'title' => $this->t('Subscribing to: %title', ['%title' => $feed->label()]),
   +      'init_message' => $this->t('Subscribing to: %title', ['%title' => $feed->label()]),
          'operations' => [
            ['Drupal\feeds\EventSubscriber\PubSubHubbub::runSubscribeBatch', [$subscription]],
          ],
   -      'progress_message' => t('Subscribing: %title', ['%title' => $feed->label()]),
   -      'error_message' => t('An error occored while subscribing to %title.', ['%title' => $feed->label()]),
   +      'progress_message' => $this->t('Subscribing: %title', ['%title' => $feed->label()]),
   +      'error_message' => $this->t('An error occored while subscribing to %title.', ['%title' => $feed->label()]),
        ];
    
        batch_set($batch);
   @@ -133,13 +133,13 @@ class PubSubHubbub implements EventSubscriberInterface {
        $subscription->unsubscribe();
    
        $batch = [
   -      'title' => t('Unsubscribing from: %title', ['%title' => $feed->label()]),
   -      'init_message' => t('Unsubscribing from: %title', ['%title' => $feed->label()]),
   +      'title' => $this->t('Unsubscribing from: %title', ['%title' => $feed->label()]),
   +      'init_message' => $this->t('Unsubscribing from: %title', ['%title' => $feed->label()]),
          'operations' => [
            ['Drupal\feeds\EventSubscriber\PubSubHubbub::runSubscribeBatch', [$subscription]],
          ],
   -      'progress_message' => t('Unsubscribing: %title', ['%title' => $feed->label()]),
   -      'error_message' => t('An error occored while unsubscribing from %title.', ['%title' => $feed->label()]),
   +      'progress_message' => $this->t('Unsubscribing: %title', ['%title' => $feed->label()]),
   +      'error_message' => $this->t('An error occored while unsubscribing from %title.', ['%title' => $feed->label()]),
        ];
   

   The PubSubHubbub class does not have a t() method. In order to get one, it should use the StringTranslationTrait.

If applying the best practises anyway, the PubSubHubbub class should also get the string translation service injected. This service can be passed by specifying it in feeds.services.yml: add an argument for the service 'feeds.pubsubhubbub'.

This got fixed in #3021003: use function t() on EventSubscriber/PubSubHubbub.php.