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/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