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>

app/views/wiki_pages/index.html.erb

@@ -5,7 +5,7 @@
 
   <%= table_for @wiki_pages, width: "100%" do |t| %>
     <% t.column "Title" do |wiki_page| %>
-      <span class="category-<%= wiki_page.category_name %>"><%= link_to_wiki wiki_page.title %></span>
+      <span class="<%= tag_class(wiki_page.tag) %>"><%= link_to_wiki wiki_page.title %></span>
     <% end %>
     <% t.column "Last edited" do |wiki_page| %>
       <%= time_ago_in_words_tagged(wiki_page.updated_at) %>

app/views/wiki_pages/show.html.erb

@@ -6,7 +6,7 @@
 
 <% content_for(:content) do %>
   <h1 id="wiki-page-title">
-    <%= link_to @wiki_page.pretty_title, posts_path(:tags => @wiki_page.title), :class => "tag-type-#{@wiki_page.category_name}" %>
+    <%= link_to @wiki_page.pretty_title, posts_path(tags: @wiki_page.title), class: tag_class(@wiki_page.tag) %>
 
     <% if @wiki_page.is_locked? %>
       (locked)