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/helpers/tags_helper.rb

@@ -0,0 +1,6 @@
+module TagsHelper
+  def tag_class(tag)
+    return nil if tag.blank?
+    "tag-type-#{tag.category}"
+  end
+end