[ticket/12714] Move "allow all forums" in its own dd-element

PHPBB3-12714
Merge pull request #2597 from Skouat/ticket/12714

[ticket/12714] Switching the description of radio buttons to align the correct description above the listbox

* Skouat/ticket/12714:
  [ticket/12714] Move "allow all forums" in its own dd-element