[ticket/12532] Add navbar_header_username_append/prepend

Use cases: add a greeting message (such as "Hello, {avatar} {username}!"), add badges after the username (such as "{avatar} {username} {badge1}"), add HTML formatting around them (such as "\<i>{avatar} {username}\</i>").

http://tracker.phpbb.com/browse/PHPBB3-12532
@imkingdavid bump?