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.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