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

app/presenters/tag_set_presenter.rb

@@ -61,6 +61,36 @@ class TagSetPresenter < Presenter
     end.join(" \n")
   end
 
+  def humanized_essential_tag_string(category_list: TagCategory.humanized_list, default: "")
+    strings = category_list.map do |category|
+      mapping = TagCategory.humanized_mapping[category]
+      max_tags = mapping["slice"]
+      regexmap = mapping["regexmap"]
+      formatstr = mapping["formatstr"]
+      excluded_tags = mapping["exclusion"]
+
+      type_tags = tags_for_category(category).map(&:name) - excluded_tags
+      next if type_tags.empty?
+
+      if max_tags > 0 && type_tags.length > max_tags
+        type_tags = type_tags.take(max_tags) + ["others"]
+      end
+
+      if regexmap != //
+        type_tags = type_tags.map { |tag| tag.match(regexmap)[1] }
+      end
+
+      if category == "copyright" && tags_for_category("character").blank?
+        type_tags.to_sentence
+      else
+        formatstr % type_tags.to_sentence
+      end
+    end
+
+    strings = strings.compact.join(" ").tr("_", " ")
+    strings.blank? ? default : strings
+  end
+
   private
 
   def tags
@@ -128,5 +158,5 @@ class TagSetPresenter < Presenter
     html
   end
 
-  memoize :tags, :tags_by_category, :ordered_tags
+  memoize :tags, :tags_by_category, :ordered_tags, :humanized_essential_tag_string
 end