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/artists/_show.html.erb

@@ -3,7 +3,7 @@
 
 <div id="c-artists">
   <div id="a-show">
-    <h1>Artist: <%= link_to @artist.pretty_name, posts_path(:tags => @artist.name), :class => "tag-type-#{@artist.category_name}" %></h1>
+    <h1>Artist: <%= link_to @artist.pretty_name, posts_path(tags: @artist.name), class: tag_class(@artist.tag) %></h1>
 
     <% if @artist.notes.present? %>
       <div class="prose">