Check meeting before displaying a join button

We also added a text change when the user is a moderator, this could probably use a new configuration text.

      $default_text = $entity->access('update') ? $this->t('Moderate Meeting') : $this->t('Join Meeting');
      $form['submit'] = [
        '#type' => 'submit',
        '#value' => $display_options["join_button_text"] ? $display_options["join_button_text"] : $default_text,
        '#attributes' => ['class' => ['btn-accent']],
      ];