TagSetPresenter: refactor humanized_essential_tag_string.

Move Post#humanized_essential_tag_string to TagSetPresenter#humanized_essential_tag_string.

This allows humanized_essential_tag_string to reuse the same set of tags
already fetched by the tag set presenter for the sidebar.

This avoids fetching the tag categories from memcache again (via
Post#typed_tags) when we're already fetched the tags once before.

This also means it's no longer necessary to cache humanized_essential_tag_string
itself in memcache, since it can be generated as quickly as the sidebar taglist.

app/presenters/post_presenter.rb

@@ -144,7 +144,7 @@ class PostPresenter < Presenter
   end
 
   def humanized_essential_tag_string
-    @post.humanized_essential_tag_string
+    @humanized_essential_tag_string ||= tag_set_presenter.humanized_essential_tag_string(default: "##{@post.id}")
   end
 
   def filename_for_download