FIX: topic_creator accepts participant_count in import mode (#10632)

The issue mentioned here: https://meta.discourse.org/t/imported-private-discussion-doesnt-appear-in-the-author-inbox/163252

`participant_count` is important to attribute for private messages. If they are imported, we should allow them to set that attribute.

A workaround would be evaluating `update_statistics` method on each Topic but that is less performant.

lib/topic_creator.rb

@@ -106,6 +106,10 @@ class TopicCreator
       topic_params[:views] = @opts[:views].to_i
     end
 
+    if topic_params[:import_mode] && @opts[:participant_count].to_i > 0
+      topic_params[:participant_count] = @opts[:participant_count].to_i
+    end
+
     # Automatically give it a moderator warning subtype if specified
     topic_params[:subtype] = TopicSubtype.moderator_warning if @opts[:is_warning]