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?