[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 
You have a period instead of a comma after 'CONTACT_USER'. I don't see the need to use get_username_string(). Simply use `$user_info['username']`.

Another issue that you'll encounter here is that we load the memberlist.php language file which also has CONTACT_USER defined. This seems to be an oversight, so the appropriate course of action here seems to be to remove CONTACT_USER from memberlist.php.