Remove theme_more_help_link() and replace with a #type link render array

Summary

theme_more_help_link() has been removed from core and replaced with a #type link render array. This theme function was only used once in core, in the forum module.

Before (D7)

  // Direct theme() call:
  theme('more_help_link', array('url' => 'admin/help/forum'));

  // Render array:
  $more_help_link = array(
    '#theme' => 'more_help_link',
    '#url' => 'admin/help/forum',
  );
  drupal_render($more_help_link);

After (D8)

  $more_help_link = array(
    '#type' => 'link',
    '#href' => 'admin/help/forum',
    '#title' => t('More help'),
  );
  $container = array(
    '#theme' => 'container',
    '#children' => drupal_render($more_help_link),
    '#attributes' => array(
      'class' => array('more-help-link'),
    ),
  );
  $output = drupal_render($container);