[ticket/11275] Do not use document.write() for color palette

This PR rewrites color palette handler. Instead of using document.write() it now inserts HTML code by element ID. Also editor.js is included via INCLUDEJS, allowing to use jQuery inside it.

TODO later in another PR: fix the rest of editor.js and posting_buttons.html after this one is merged.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11275