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/logical/storage_manager.rb

@@ -115,7 +115,7 @@ class StorageManager
   def seo_tags(post)
     return "" if !tagged_filenames
 
-    tags = post.humanized_essential_tag_string.gsub(/[^a-z0-9]+/, "_").gsub(/(?:^_+)|(?:_+$)/, "").gsub(/_{2,}/, "_")
+    tags = post.presenter.humanized_essential_tag_string.gsub(/[^a-z0-9]+/, "_").gsub(/(?:^_+)|(?:_+$)/, "").gsub(/_{2,}/, "_")
     "__#{tags}__"
   end
 end