[ticket/12525] Add a new lang var and introduce args for lang vars for memberlist

In common.php:
* Introduce a new language variable 'CONTACT' => 'Contact'
* Add a string argument to lang var 'CONTACT_USER' to avoid an appended
 '{USERNAME}' in template.
* Add a string argument to lang var 'SEND_EMAIL_USER' to avoid an appended
 '{USERNAME}' in template.
* fix typos

PHPBB3-12525