Move item_link partial caching to a helper rather than the template itself

This is a surprisingly huge performance gain. On my testing (with
cache_classes set to true to also cache templates), this sped up
closet_hangers#index rendering by a factor of 2 when there were a
significant number of items. Cool beans.

I think we can even hold off on the individual hanger caching now:
we've made the closet hanger partial tons faster by moving forms out
of them and doing this cache check earlier. I'm expecting significant
performance gains both here and on items#index (though less so there).
I'll deploy and see how much it helps in production; if not enough, we
can look at the layered caching of hangers, lists, groups, full pages,
etc.

So glad we don't *have* to move to a pagination model!

app/helpers/items_helper.rb

@@ -120,6 +120,23 @@ module ItemsHelper
     " (×#{count})".html_safe if count > 1
   end
 
+  def render_item_link(item)
+    # I've discovered that checking the cache *before* attempting to render the
+    # partial is significantly faster: moving the cache line out here instead
+    # of having it wrap the partial's content speeds up closet_hangers#index
+    # rendering time by about a factor of 2. It's uglier, but this call happens
+    # a lot, so the performance gain is definitely worth it. I'd be interested
+    # in a more legit partial caching abstraction, but, for now, this will do.
+    # Because this is a returned-string helper, but uses a buffer-output
+    # helper, we have to do some indirection. Fake that the render is in a
+    # template, then capture the resulting buffer output.
+    capture do
+      localized_cache("items/#{item.id}#item_link_partial") do
+        safe_concat render(partial: 'items/item_link', locals: {item: item})
+      end
+    end
+  end
+
   private
 
   def build_on_pet_types(species, special_color=nil, &block)

app/views/closet_hangers/_closet_hanger.html.haml

@@ -1,4 +1,4 @@
 %div{'class' => closet_hanger_partial_class(closet_hanger), 'data-item-id' => closet_hanger.item_id, 'data-quantity' => closet_hanger.quantity, 'data-id' => closet_hanger.id}
-  = render :partial => 'items/item_link', :locals => {:item => closet_hanger.item}
+  = render_item_link(closet_hanger.item)
   .quantity{:class => "quantity-#{closet_hanger.quantity}"}
     %span= closet_hanger.quantity

app/views/items/_item.html.haml

@@ -1,3 +1,3 @@
 .object
-  = render :partial => 'items/item_link', :locals => {:item => item}
+  = render_item_link(item)
   = closeted_icons_for(item)

app/views/items/_item_link.html.haml

@@ -1,5 +1,4 @@
-- localized_cache "items/#{item.id}#item_link_partial" do
+= link_to item_path(item) do
-  = link_to item_path(item) do
+  = image_tag item.thumbnail_url, :alt => item.description, :title => item.description
-    = image_tag item.thumbnail_url, :alt => item.description, :title => item.description
+  %span.name= item.name
-    %span.name= item.name
+  = nc_icon_for(item)
-    = nc_icon_for(item)