Stack notifications everywhere

We had client-side notification stacking in the tray since launch, but now we want to stack notifications in the notifications page, in API responses and in email bundles.

test/models/user/notifiable_test.rb

@@ -3,6 +3,7 @@ require "test_helper"
 class User::NotifiableTest < ActiveSupport::TestCase
   setup do
     @user = users(:david)
+    @user.notifications.destroy_all
     @user.settings.bundle_email_every_few_hours!
   end
 
@@ -12,7 +13,7 @@ class User::NotifiableTest < ActiveSupport::TestCase
     end
 
     bundle = @user.notification_bundles.last
-    assert_equal notification.created_at, bundle.starts_at
+    assert_equal notification.updated_at, bundle.starts_at
     assert bundle.pending?
   end
 
@@ -20,7 +21,7 @@ class User::NotifiableTest < ActiveSupport::TestCase
     @user.notifications.create!(source: events(:logo_published), creator: @user)
 
     assert_no_difference -> { @user.notification_bundles.count } do
-      @user.notifications.create!(source: events(:logo_published), creator: @user)
+      @user.notifications.create!(source: events(:layout_published), creator: @user)
     end
   end
 end