[ticket/15735] Add posting_preview_content_after template event

Needed for extensions who adds information at the end of the message
(such as moderator messages).

PHPBB3-15735

phpBB/styles/prosilver/template/posting_preview.html

@@ -31,6 +31,8 @@
 
 		<div class="content">{PREVIEW_MESSAGE}</div>
 
+		<!-- EVENT posting_preview_content_after -->
+
 		<!-- IF .attachment -->
 		<dl class="attachbox">
 			<dt>{L_ATTACHMENTS}</dt>