[Ticket/12680] - Adds alternative text to contact icon in viewtopic.php

Similar to the edit, quote, delete.... icons the new contact icon under the "joined" line, there a added some alternative text, to display when CSS is disabled. 

PHPBB3-12426 https://tracker.phpbb.com/browse/PHPBB3-12680 
This should be `$user_cache[$poster_id]['contact_user']`.