TagSetPresenter: refactor *_tag_list_html to avoid memcache calls.

Refactor the tag set presenter to get both the tag categories and the
tag counts in the same call to the database, instead of getting the
counts from the db and the categories from memcache.

app/models/tag.rb

@@ -56,10 +56,6 @@ class Tag < ApplicationRecord
     extend ActiveSupport::Concern
 
     module ClassMethods
-      def counts_for(tag_names)
-        select_all_sql("SELECT name, post_count FROM tags WHERE name IN (?)", tag_names)
-      end
-
       def highest_post_count
         Cache.get("highest-post-count", 4.hours) do
           select("post_count").order("post_count DESC").first.post_count