users: drop id_to_name, name_to_id caching.

Changes:

* Drop Users.id_to_name.
* Don't cache Users.name_to_id.
* Replace calls to name_to_id with find_by_name when possible.
* Don't autodefine creator_name in belongs_to_creator.
* Don't autodefine updater_name in belongs_to_updater.
* Instead manually define creator_name / updater_name only on models that need
  to return these fields in the api.

id_to_name was cached to reduce the impact of N+1 query patterns in
certain places, especially in api responses that return creator_name /
updater_name fields. But it still meant we were doing N calls to
memcache. Using `includes` to prefetch users avoids this N+1 pattern.

name_to_id had no need be cached, it was never used in any performance-
sensitive contexts.

Avoiding caching also avoids the need to keep these caches consistent.

app/views/posts/index.atom.erb

@@ -29,7 +29,7 @@
         </div>
       </content>
       <author>
-        <name><%= post.uploader_name %></name>
+        <name><%= post.uploader.name %></name>
       </author>
     </entry>
   <% end %>

app/views/posts/partials/show/_image.html.erb

@@ -1,3 +1,3 @@
 <% if post.visible? %>
-  <%= image_tag(post.file_url_for(CurrentUser.user), :width => post.image_width_for(CurrentUser.user), :height => post.image_height_for(CurrentUser.user), :id => "image", "data-original-width" => post.image_width, "data-original-height" => post.image_height, "data-large-width" => post.large_image_width, "data-large-height" => post.large_image_height, "data-tags" => post.tag_string, :alt => post.presenter.humanized_essential_tag_string, "data-uploader" => post.uploader_name, "data-rating" => post.rating, "data-flags" => post.status_flags, "data-parent-id" => post.parent_id, "data-has-children" => post.has_children?, "data-has-active-children" => post.has_active_children?, "data-score" => post.score, "data-fav-count" => post.fav_count, "itemprop" => "contentUrl") %>
+  <%= image_tag(post.file_url_for(CurrentUser.user), :width => post.image_width_for(CurrentUser.user), :height => post.image_height_for(CurrentUser.user), :id => "image", "data-original-width" => post.image_width, "data-original-height" => post.image_height, "data-large-width" => post.large_image_width, "data-large-height" => post.large_image_height, "data-tags" => post.tag_string, :alt => post.presenter.humanized_essential_tag_string, "data-uploader" => post.uploader.name, "data-rating" => post.rating, "data-flags" => post.status_flags, "data-parent-id" => post.parent_id, "data-has-children" => post.has_children?, "data-has-active-children" => post.has_active_children?, "data-score" => post.score, "data-fav-count" => post.fav_count, "itemprop" => "contentUrl") %>
 <% end %>

app/views/posts/partials/show/_ugoira_original.html.erb

@@ -9,7 +9,7 @@
   "data-large-width" => post.image_width,
   "data-large-height" => post.image_height,
   "data-tags" => post.tag_string,
-  "data-uploader" => post.uploader_name,
+  "data-uploader" => post.uploader.name,
   "data-rating" => post.rating,
   "data-flags" => post.status_flags,
   "data-parent-id" => post.parent_id,

app/views/posts/show.html.erb

@@ -120,7 +120,7 @@
         <% if !CurrentUser.user.is_builder? %>
           <h2>Before commenting, read the <%= link_to "how to comment guide", wiki_pages_path(:search => {:title => "howto:comment"}) %>.</h2>
         <% end %>
-        <%= render "comments/partials/index/list", :comments => @post.comments.visible(CurrentUser.user), :post => @post, :show_header => false %>
+        <%= render "comments/partials/index/list", :comments => @post.comments.visible(CurrentUser.user).includes(:creator), :post => @post, :show_header => false %>
       </section>
 
       <section id="notes" style="display: none;">