views: replace .category-N css classes with .tag-type-N

* Replace the .category-N CSS classes on tags with .tag-type-N. Before
  we were inconsistent about whether tag colors were indicated with
  .category-N or .tag-type-N. Now it's always .tag-type-N.

* Fix various places to not use Tag.category_for. Tag.category_for does
  one Redis call per tag lookup, which leads to N Redis calls on many
  pages. This was inefficient because usually we either already had the
  tags from the database, or we could fetch them easily.

app/models/wiki_page_version.rb

@@ -3,6 +3,7 @@ class WikiPageVersion < ApplicationRecord
   belongs_to :wiki_page
   belongs_to_updater
   belongs_to :artist, optional: true
+  belongs_to :tag, primary_key: :name, foreign_key: :title, optional: true
 
   module SearchMethods
     def search(params)
@@ -51,10 +52,6 @@ class WikiPageVersion < ApplicationRecord
     !is_deleted && previous.is_deleted
   end
 
-  def category_name
-    Tag.category_for(title)
-  end
-
   def self.available_includes
     [:updater, :wiki_page, :artist]
   end