[ticket/12247] Add ['username'] to mcp_queue.php's user_notification()

It includes the poster's username in the email notifications of
posts that get approved by moderators.

This is done by adding the username to every user_notification()
function located in /phpBB/includes/mcp/mcp_queue.php.

PHPBB3-12247
[ticket/12247] Makes static usernames work

Relative to the last ticket, this allows static usernames to
work correctly (those inserted in the post-column post_username)
when sending the e-mail, instead of showing just "Anonymous".

PHPBB3-12247
Merge remote-tracking branch 'marcosbc/ticket/12247' into develop-olympus

* marcosbc/ticket/12247:
  [ticket/12247] Makes static usernames work
  [ticket/12247] Add ['username'] to mcp_queue.php's user_notification()