Test digest user selector includes anonymous

Problem/Motivation

The user autocomplete needs to be told not to list the anonymous user as you can can't send a digest to them :)

Steps to reproduce

Proposed resolution

Change

    $form['send_to_user'] = [
      '#type' => 'entity_autocomplete',
      '#target_type' => 'user',
      '#title' => $this->t('Send to user'),
      '#description' => $this->t('The digest will be sent out to this user.'),
      '#default_value' => $user_storage->load($this->currentUser->id()),
    ];

to

    $form['send_to_user'] = [
      '#type' => 'entity_autocomplete',
      '#target_type' => 'user',
      '#title' => $this->t('Send to user'),
      '#description' => $this->t('The digest will be sent out to this user.'),
      '#default_value' => $user_storage->load($this->currentUser->id()),
      '#selection_settings' => [
        'include_anonymous' => FALSE,
      ],
    ];

Remaining tasks

User interface changes

API changes

Data model changes