[WIP][ticket/12647] Include jQuery UI in core

Using method from @nickvergessen : `<!-- IF $LOAD_JQUERY_UI -->` which extensions can use by including `<!-- DEFINE $LOAD_JQUERY_UI = 'true' -->` anywhere EXCEPT `<!-- EVENT overall_footer_after -->` (will be too late).

Also supports CDN

https://tracker.phpbb.com/browse/PHPBB3-12647
https://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=44923

PHPBB3-12647