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/views/wiki_pages/_recent_changes.html.erb

@@ -1,8 +1,8 @@
 <section>
   <h1>Recent Changes (<%= link_to "all", wiki_page_versions_path %>)</h1>
   <ul>
-    <% WikiPage.recent.each do |page| %>
-      <li class="category-<%= page.category_name %>"><%= link_to page.pretty_title, wiki_page_path(page) %></li>
+    <% WikiPage.order(updated_at: :desc).includes(:tag).limit(25).each do |page| %>
+      <li><%= link_to page.pretty_title, wiki_page_path(page), class: tag_class(page.tag) %></li>
     <% end %>
   </ul>
 </section>