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/controllers/dtext_links_controller.rb

@@ -3,7 +3,7 @@ class DtextLinksController < ApplicationController
 
   def index
     @dtext_links = DtextLink.paginated_search(params)
-    @dtext_links = @dtext_links.includes(:model) if request.format.html?
+    @dtext_links = @dtext_links.includes(linked_wiki: :tag, model: :tag) if request.format.html?
 
     respond_with(@dtext_links)
   end