[ticket/12525] Add string argument to contact and email user language variables

In language/en/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.

Also fix typos.

PHPBB3-12525