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/artist.rb

@@ -354,10 +354,6 @@ class Artist < ApplicationRecord
   end
 
   module TagMethods
-    def category_name
-      Tag.category_for(name)
-    end
-
     def validate_tag_category
       return unless is_active? && name_changed?
 

app/models/dtext_link.rb

@@ -1,5 +1,7 @@
 class DtextLink < ApplicationRecord
   belongs_to :model, polymorphic: true
+  belongs_to :linked_wiki, primary_key: :title, foreign_key: :link_target, class_name: "WikiPage", optional: true
+
   enum link_type: [:wiki_link, :external_link]
 
   before_validation :normalize_link_target

app/models/wiki_page.rb

@@ -38,10 +38,6 @@ class WikiPage < ApplicationRecord
       where("is_deleted = false")
     end
 
-    def recent
-      order("updated_at DESC").limit(25)
-    end
-
     def other_names_include(name)
       name = normalize_other_name(name)
       subquery = WikiPage.from("unnest(other_names) AS other_name").where_iequals("other_name", name)
@@ -177,7 +173,7 @@ class WikiPage < ApplicationRecord
   end
 
   def category_name
-    Tag.category_for(title)
+    tag&.category
   end
 
   def pretty_title

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