[ticket/12374] Add Template events index_body_block_(name)_append

Just a simple append. There are mods that would like to add very simple data to these blocks. Such as adding the currently "chatting" users to the online list. Or adding the number of "active" users to the statistics block.

All this without having to create entire new blocks (and thereby avoid screen clutter)
http://tracker.phpbb.com/browse/PHPBB3-12374

PHPBB3-12374