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/javascript/src/styles/common/tags.scss.erb

@@ -1,7 +1,7 @@
 <% TagCategory.css_mapping.each do |category,cssmap| %>
-  .category-<%= category %> a,
+  .tag-type-<%= category %> a,
   a.tag-type-<%= category %>,
-  .ui-widget-content .category-<%= category %> a,
+  .ui-widget-content .tag-type-<%= category %> a,
   .ui-widget-content a.tag-type-<%= category %> {
     color: <%= cssmap["color"] %>;