[ticket/11809] Load core.js directly after jQuery instead of using INCLUDEJS

The file core.js sets up most of the AJAX and jQuery related framework for
phpBB. Due to this, it needs to be included right after jQuery has been
included. Currently, it is however loaded with INCLUDEJS which causes it to
be loaded after every other javascript file except for the style specific
ajax.js. This causes every javascript file that is included before core.js
and that is using the phpBB AJAX functions or variables to break.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11809

PHPBB3-11809