[ticket/11563] Fix subPanels()

Fix subPanels() code
Modernize subPanels() with jQuery
Use HTML5 data attributes instead of including JS

PHPBB3-11563
[ticket/11563] Remove unused JS variables

Remove unused JS variables from posting_buttons

PHPBB3-11563
[ticket/11563] Dynamically generate panels list

PHPBB3-11563
[ticket/11563] Remove duplicate code

PHPBB3-11563
Merge pull request #1436 from cyberalien/ticket/11563

Fix subPanels() in forum_fn.js