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

In viewtopic_body.html we have:

````<!-- EVENT viewtopic_body_postrow_post_before -->
<!-- EVENT viewtopic_body_postrow_post_after -->````

I propose:
````<!-- EVENT search_results_post_before -->
<!-- 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