FIX: Limit personal message participants when converting from topic (#9343)

Previously all topic posters would be added which could lead to major performance issues. Now if there are too many posters, only the acting user will be added as a participant.

spec/models/topic_converter_spec.rb

@@ -152,6 +152,18 @@ describe TopicConverter do
         expect(Notification.exists?(user_notification.id)).to eq(false)
         expect(Notification.exists?(admin_notification.id)).to eq(true)
       end
+
+      it "limits PM participants" do
+        SiteSetting.max_allowed_message_recipients = 2
+        Fabricate(:post, topic: topic)
+        Fabricate(:post, topic: topic)
+
+        private_message = topic.convert_to_private_message(post.user)
+
+        # Skips posters and just adds the acting user
+        expect(private_message.topic_allowed_users.count).to eq(1)
+      end
+
     end
 
     context 'topic has replies' do