[ticket/9550] Add template event ucp_pm_viewmessage_postprofile_append

Adds a template event required for the karma extension. It allows adding data
under the username and avatar next to every private message in prosilver.

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 users
to see how respected the pm sender is.

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

Adds the prepend counterpart of a template event required for the karma
extension. It allows adding data under the username and avatar next to every
private message in prosilver.

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