Properly render menu trees in a menu_block when using drupal-bootstrap subtheme (Drupal 7, bootstrap 7.x-3.x, menu_block)

template.php

<?php
/**
 * Implements theme_menu_link() for the main menu when displayed in menu_block
 * block.
 * 
 * Stock bootstrap theme doesn't render menus deeper than 2 levels because the
 * navbar feature doesn't support a third level. But when rendering the menu in
 * the sidebar, we can do unlimited nesting because we're not using the dynamic
 * navbar.  
 *
 * This is only useful in a subtheme of bootstrap (tested with 7.x-3.0); you
 * might need to adjust the name of the theme override to match your needs.
 *
 * We crib from bootstrap_menu_link() and theme_menu_link().
 */
function THEME_menu_link__menu_block__main_menu(array &$variables) {
  $element = $variables['element'];
  $sub_menu = '';

  if ($element['#below']) {
    $sub_menu = drupal_render($element['#below']);
  }

  // Add "has-children" class to links which have children
  if ($element['#original_link']['has_children']) {
    $element['#attributes']['class'][] = 'has-children';
  }

  // On primary navigation menu, class 'active' is not set on active menu item.
  // @see https://drupal.org/node/1896674
  if (($element['#href'] == $_GET['q'] || ($element['#href'] == '<front>' && drupal_is_front_page())) && (empty($element['#localized_options']['language']))) {
    $element['#attributes']['class'][] = 'active';
  }
  $output = l($element['#title'], $element['#href'], $element['#localized_options']);
  return '<li' . drupal_attributes($element['#attributes']) . '>' . $output . $sub_menu . "</li>\n";
}