[ticket/11508] Build the jumpbox hidden fields using the form action

The current code builds the jumpbox hidden fields based on the URL of the current page. This causes multiple problems as the current page is almost never the same as the jumpbox action. This also passes on unnecessary parameters such as start, t, quickmod and others. The goal with that was to preserve persistent parameters such as style. This was unnecessary since append_sid() already takes care of this. The sid is also excluded from the hidden fields which results in the user getting logged out after using the jumpbox if cookies are disabled.

http://tracker.phpbb.com/browse/PHPBB3-11508