New 'hook_social_core_default_main_menu_links_alter()' hook

We added the ability to alter the main menu links that ships with Open Social. Developers can add/remove links via a new hook hook_social_core_default_main_menu_links_alter()

An example of how to add a new link:

/**
 * Implements hook_social_core_default_main_menu_links_alter().
 */
function hook_social_core_default_main_menu_links_alter(array &$links) {
  $link = \Drupal::entityTypeManager()->getStorage('menu_link_content')
    ->loadByProperties([
      'title' => 'Community',
      'menu_name' => 'main',
      'link' => 'internal:/explore',
    ]);
  $link = end($link);

  $links[] = $link;
}