[ticket/11311] Include asset core.js in subsilver2 overall_footer.html

Without the inclusion of core.js the timezone functions will not properly
work. They refer to phpbb which is defined in core.js.

PHPBB3-11311
Merge remote-tracking branch 'marc1706/ticket/11311' into develop

* marc1706/ticket/11311:
  [ticket/11311] Include asset core.js in subsilver2 overall_footer.html