[ticket/12410] Add Template events search_results_post_ b/a

In viewtopic_body.html we have:

`<!-- EVENT viewtopic_body_postrow_post_before -->` and `<!-- EVENT viewtopic_body_postrow_post_after -->`

I propose:
`<!-- EVENT search_results_post_before -->` and `<!-- EVENT search_results_post_after -->`

This will allow us to do similar things with the search results.

https://tracker.phpbb.com/browse/PHPBB3-12410
PHPBB3-12410