[ticket/12333] Add Template Event overall_header_page_body_before

Recently, the EVENT overall_header_body_before has been added.

I propose the EVENT overall_header_body_after.

Besides symmetry, it is useful to be able to insert code BETWEEN the \<div id="page-header"> and \<div id="page-body"> elements (this is currently not possible).

PHPBB3-12333