[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
I would vote to take <!-- EVENT navbar_header_logged_out_content --> out of this ```<li>``` and put into a new one when !S_REGISTERED_USER so that we dont add any empty ```<li>```