WIP add search index method to landing page

This is needed because the Attachable module assumes that the class its
attached to has a search_index method, which landing page doesn't. This
usually comes from the Searchable module, but we don't need any of that
behaviour.

Actually we don't need Attachable either, I don't think - I think it's a
hangover from when we were attaching CSVs to use for line graphs. But
we're not doing line graphs now. We could remove it, but I'm slightly
nervous about breaking something.

app/models/landing_page.rb

@@ -41,6 +41,10 @@ def landing_page_body
     end
   end
 
+  def search_index
+    {}
+  end
+
 private
 
   def base_path_must_not_be_taken