DEV: Improve support for plugins. (#7332)

* DEV: Allow NewPostManager handlers handle PMs.
* DEV: Add custom fields topic option to PostCreator.
* DEV: Add topic_id to serializer data.
* DEV: Wrap topic title from notification item in a span.

lib/topic_creator.rb

@@ -37,6 +37,9 @@ class TopicCreator
   def create
     topic = Topic.new(setup_topic_params)
     setup_tags(topic)
+    if fields = @opts[:custom_fields]
+      topic.custom_fields = fields
+    end
 
     DiscourseEvent.trigger(:before_create_topic, topic, self)