Recreate Author Pane functionality on comment display

Some details that are needed:

/**
 * Implements hook_entity_extra_field_info().
 */
function forum_entity_extra_field_info() {
  $fields['user']['user']['display']['current_status'] = [
    'label' => t('Current status'),
    'description' => t("Whether the user is currently logged in."),
    'weight' => 3,
  ];
  $fields['user']['user']['display']['username'] = [
    'label' => t('Username'),
    'description' => t("The user's raw username."),
    'weight' => 4,
  ];
  $fields['user']['user']['display']['joined'] = [
    'label' => t('Joined'),
    'description' => t("When the user joined the site."),
    'weight' => 6,
  ];
  $fields['user']['user']['display']['last_seen'] = [
    'label' => t('Last seen'),
    'description' => t("When the user was last seen on the site."),
    'weight' => 7,
  ];
  $fields['user']['user']['display']['contact_link'] = [
    'label' => t('Contact link'),
    'description' => t("Link to the user's contact form."),
    'weight' => 8,
  ];

  return $fields;
}

/**
 * Implements hook_ENTITY_TYPE_view() for user entities.
 */
function forum_user_view(array &$build, UserInterface $account, EntityViewDisplayInterface $display) {
  if ($account->isAuthenticated()) {
    if ($display->getComponent('current_status')) {
      $build['current_status'] = [
        '#type' => 'item',
        '#markup' => $account->isActive() ? t('active') : t('inactive'),
      ];
    }
    if ($display->getComponent('username')) {
      $username = [
        '#theme' => 'username',
        '#account' => $account,
      ];
      $build['username'] = [
        '#type' => 'item',
        '#markup' => \Drupal::service('renderer')->render($username),
      ];
    }
    if ($display->getComponent('joined')) {
      $build['joined'] = [
        '#type' => 'item',
        '#markup' => '<p><span class="label">' . t('Joined:') . '</span> '
        . \Drupal::service('date.formatter')->format($account->getLastLoginTime()) . '</p>',
      ];
    }
    if ($display->getComponent('last_seen')) {
      $build['last_seen'] = [
        '#type' => 'item',
        '#markup' => '<p><span class="label">' . t('Last seen:') . '</span> '
        . \Drupal::service('date.formatter')->formatTimeDiffSince($account->getLastLoginTime()) . '</p>',
      ];
    }
    if ($display->getComponent('contact_link')) {
      $build['contact_link'] = [
        '#type' => 'item',
        '#markup' => '<p>' . $account->toLink('Contact', 'contact-form')->toString() . '</p>',
      ];
    }
  }
}

WIP.

#4 was missing some use statements, this should work.

To make the display work I've found that adding the fields to the "compact" view mode is a start, then making changes to the comment.html.twig template as needed.

Moving the work back to the primary ticket.