Introduce Whitehall.attachment_notifier

This is intended to work in a similar way to the notifier in
EditionServiceCoordinator, except that it's used to publish events
concerning the Attachment class. My plan is to subscribe an
instance of ServiceListeners::AttachmentDraftStatusUpdater to this
notifier in order to keep the draft status of attachments associated
with policy groups up-to-date in Asset Manager.

Note that the visibility of attachments on a policy group is
determined simply by the existence of the policy group. This logic is
already encapsulated in the AttachmentVisibility class which is used in
the ServiceListeners::AttachmentDraftStatusUpdater.

I did consider introducing a service for creating attachments, but there
doesn't seem to be any precedent for using services in that way and in
any case it felt a bit like overkill.

I also considered wiring an instance of
ServiceListeners::AttachmentDraftStatusUpdater directly into the
Admin::AttachmentsController, but that felt at odds with the approach
taken elsewhere and would've introduced unnecessary coupling and made
testing of the controller harder.

Note that there's no need to publish 'update' events as well as 'create'
events, because updating an attachment should not have any effect on the
draft status of its corresponding asset. We might need to introduce a
'destroy' event at some point, but we're planning to tackle that
separately in this issue [1].

[1]: alphagov/asset-manager#469

app/controllers/admin/attachments_controller.rb

@@ -17,6 +17,7 @@ def new; end
 
   def create
     if save_attachment
+      Whitehall.attachment_notifier.publish('create', attachment)
       redirect_to attachable_attachments_path(attachable), notice: "Attachment '#{attachment.title}' uploaded"
     else
       render :new

lib/whitehall.rb

@@ -153,6 +153,10 @@ def self.edition_services
     @edition_actions ||= EditionServiceCoordinator.new
   end
 
+  def self.attachment_notifier
+    @attachment_notifier ||= ActiveSupport::Notifications::Fanout.new
+  end
+
   def self.organisations_in_tagging_beta
     @taggable_organisations ||=
       YAML.load_file(Rails.root + "config/organisations_in_tagging_beta.yml")["organisations_in_tagging_beta"]

test/functional/admin/attachments_controller_test.rb

@@ -29,6 +29,7 @@ def valid_external_attachment_params
   setup do
     login_as :gds_editor
     @edition = create(:consultation)
+    Whitehall.attachment_notifier.stubs(:publish)
   end
 
   def self.supported_attachable_types
@@ -93,6 +94,20 @@ def self.supported_attachable_types
     assert_redirected_to admin_edition_attachments_url(attachable)
   end
 
+  test "POST :create publishes event to attachment notifier on success" do
+    Whitehall.attachment_notifier.expects(:publish).with('create', is_a(Attachment))
+    attachable = create(:edition)
+
+    post :create, params: { edition_id: attachable.id, attachment: valid_file_attachment_params }
+  end
+
+  test "POST :create does not publish event to attachment notifier on failure" do
+    Whitehall.attachment_notifier.expects(:publish).never
+    attachable = create(:edition)
+
+    post :create, params: { edition_id: attachable.id, attachment: {} }
+  end
+
   view_test 'GET :index shows html attachments' do
     create(:html_attachment, title: 'An HTML attachment', attachable: @edition)