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.

spec/components/new_post_manager_spec.rb

@@ -212,7 +212,7 @@ describe NewPostManager do
       handler = -> { nil }
 
       NewPostManager.add_handler(&handler)
-      expect(NewPostManager.handlers).to eq([default_handler, handler])
+      expect(NewPostManager.handlers).to eq([handler])
     end
 
     it "can be added in high priority" do
@@ -223,7 +223,7 @@ describe NewPostManager do
       NewPostManager.add_handler(100, &a)
       NewPostManager.add_handler(50, &b)
       NewPostManager.add_handler(101, &c)
-      expect(NewPostManager.handlers).to eq([c, a, b, default_handler])
+      expect(NewPostManager.handlers).to eq([c, a, b])
     end
 
   end

spec/components/post_creator_spec.rb

@@ -54,6 +54,11 @@ describe PostCreator do
       expect { creator.create }.to raise_error(Discourse::InvalidAccess)
     end
 
+    it "can be created with custom fields" do
+      post = PostCreator.create(user, basic_topic_params.merge(topic_opts: { custom_fields: { hello: "world" } }))
+      expect(post.topic.custom_fields).to eq("hello" => "world")
+    end
+
     context "reply to post number" do
       it "omits reply to post number if received on a new topic" do
         p = PostCreator.new(user, basic_topic_params.merge(reply_to_post_number: 3)).create