Use a placeholder image from asset-manager

In the past this linked to an image served by Whitehall, but because of
the way Rails generates hashes for assets, we can't guarantee that this
URL will always exist. This hasn't been a problem in the past because
we've been able to put an unhashed version of the image on the Whitehall
machines, but since moving to AWS the machines can be recreated at any
moment and won't have the file.

To solve this problem I've uploaded the file to Asset Manager and we can
link to that instead. Ideally, we wouldn't depend on a file in a
different application, but this solves the problem for now.

app/presenters/content_item/news_image.rb

@@ -12,7 +12,8 @@ def default_news_image
     end
 
     def placeholder_image
-      { "url" => "https://assets.publishing.service.gov.uk/government/assets/placeholder.jpg" }
+      # this image has been uploaded to asset-manager
+      { "url" => "https://assets.publishing.service.gov.uk/media/5e59279b86650c53b2cefbfe/placeholder.jpg" }
     end
   end
 end

test/presenters/content_item/news_image_test.rb

@@ -34,7 +34,7 @@ def initialize
 
   test "presents a placeholder image if document has no image or default news image" do
     item = DummyContentItem.new
-    placeholder_image = { "url" => "https://assets.publishing.service.gov.uk/government/assets/placeholder.jpg" }
+    placeholder_image = { "url" => "https://assets.publishing.service.gov.uk/media/5e59279b86650c53b2cefbfe/placeholder.jpg" }
 
     assert_equal placeholder_image, item.image
   end