[ticket/9550] Add template event memberlist_view_column2_details_prepend

Adds a template event required for the karma extension. It allows adding
entries to the column2 part of any user profile.

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 this user is. Prepend because karma score is often
a more meaningful statistic than the bottom statistics (like "Most active
topic"), and should therefore be more prominent.

PHPBB3-9550