[ticket/9550] Add template event memberlist_body_posts_append

Adds a template event required for the karma extension. It allows adding
information to the posts column of the memberlist.

Explanation from http://area51.phpbb.com/phpBB/viewtopic.php?f=111&t=44379:
Used by the karma extension to add a karma score indication that allows other
users to see how respected every listed user is.

PHPBB3-9550
[ticket/9550] Add template event memberlist_body_posts_prepend

Adds the prepend counterpart of a template event required for the karma
extension. It allows adding information to the posts column of the memberlist.

Explanation from http://area51.phpbb.com/phpBB/viewtopic.php?f=111&t=44379:
Per suggestion of nickvergessen, add a counterpart for every append or prepend
event.

PHPBB3-9550